Darwinism : an exposition of the theory of natural selection, with some of its applications

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DAEWINISM

DARWINISM

AN EXPOSITION OF THE

THEORY OF NATURAL SELECTION

WITH SOME OF ITS APPLICATIONS

BY

ALFBED EUSSEL WALLACE

LL.D., F.L.S., ETC.

WITH A PORTRAIT OF THE AUTHOR, MAP AND ILLUSTRATIONS

SECOND EDITION

ILonUon

MACMILLAN AND CO.

AND NEW YORK
1889

All rights reserved

First Edition published May \i

Reprinted August i8Sg

PREFACE TO SECOND EDITION

The present edition is a reprint of the first, with a few verbal
corrections and the alteration of some erroneous or doubtful
statements. Of these latter the following are the most
important : —

P. 3~0. The statement as to the fulmar petrel, which
Professor A. Newton assures me is erroneous,
has been modified.

P. 34. A note is added as to Darwin's statement about
the missel and song-thrushes in Scotland.

P. 172. An error as to the differently-coloured herds of
cattle in the Falkland Islands, is corrected.

Parkstone, Dorset
August, 1889.

PREFACE TO FIRST EDITION

The present work treats the problem of the Origin of Species
on the same general lines as were adopted by Darwin ; but
from the standpoint reached after nearly thirty years of
discussion, with an abundance of new facts and the advocacy
of many new or old theories.

While' not attempting to deal, even in outline, with the
vast subject of evolution in general, an endeavour has been
made to give such an account of the theory of Natural Selec-
tion as may enable any intelligent reader to obtain a clear
conception of Darwin's work, and to understand something
of the power and range of his great principle.

Darwin wrote for a generation which had not accepted
evolution, and which poured contempt on those who upheld
the derivation of species from species by any natural law of
descent. He did his work so well that "descent with
modification " is now universally accepted as the order of
nature in the organic world ; and the rising generation of
naturalists can hardly realise the novelty of this idea, or that
their fathers considered it a scientific heresy to be condemned
rather than seriously discussed.

The objections now made to Darwin's theory apply, solely,
to the particular means by which the change of species has
been brought about, not to the fact of that change. The
objectors seek to minimise the agency of natural selection
and to subordinate it to laws of variation, of use and disuse,
of intelligence, and of heredity. These views and objections

VI PREFACE

are urged with much force and more confidence, and for the
most part by the modern school of laboratory naturalists, to
whom the peculiarities and distinctions of species, as such,
their distribution and their affinities, have little interest as
compared with the problems of histology and embryology,
of physiology and morphology. Their work in these depart-
ments is of the greatest interest and of the highest importance,
but it is not the kind of work which, by itself, enables one to
form a sound judgment on the questions involved in the
action of the law of natural selection. These rest mainly on
the external and vital relations of species to species in a state
of nature — on what has been well termed by Semper the
"physiology of organisms," rather than on the anatomy or
physiology of organs.

It has always been considered a weakness in Darwin's
work that he based his theory, primarily, on the evidence of
variation in domesticated animals and cultivated plants. I
have endeavoured to secure a firm foundation for the theory
in the variations of organisms in a state of nature ; and as
the exact amount and precise character of these variations is
of paramount importance in the numerous problems that
arise when we apply the theory to explain the facts of nature,
I have endeavoured, by means of a series of diagrams, to
exhibit to the eye the actual variations as they are found to
exist in a sufficient number of species. By doing this, not
only does the reader obtain a better and more precise idea of
variation than can be given by any number of tabular state-
ments or cases of extreme individual variation, but we obtain
a basis of fact by which to test the statements and objections
usually put forth on the subject of specific variability ; and it
will be found that, throughout the work, I have frequently to
appeal to these diagrams and the facts they illustrate, just as
Darwin was accustomed to appeal to the facts of variation
among dogs and pigeons.

PREFACE vii

I have also made what appears to me an important change
in the arrangement of the subject. Instead of treating first
the comparatively difficult and unfamiliar details of variation,
I commence with the Struggle for Existence, which is really
the fundamental phenomenon on which natural selection
depends, while the particular facts which illustrate it are
comparatively familiar and very interesting. It has the
further advantage that, after discussing variation and the
effects of artificial selection, we proceed at once to explain
how natural selection acts.

Among the subjects of novelty or interest discussed in this
volume, and which have important bearings on the theory of
natural selection, are : (1) A proof that all specific characters
are (or once have been) either useful in themselves or cor-
related with useful characters (Chap. VI) ; (2) a proof that
natural selection can, in certain cases, increase the sterility of
crosses (Chap. VII) ; (3) a fuller discussion of the colour
relations of animals, with additional facts and arguments on
the origin of sexual differences of colour (Chaps. VIII-X) ;
(4) an attempted solution of the difficulty presented by the
occurrence of both very simple and very complex modes of
securing the cross-fertilisation of plants (Chap. XI) ; (5) some
fresh facts and arguments on the wind-carriage of seeds, and
its bearing on the wide dispersal of many arctic and alpine
plants (Chap. XII) ; (6) some new illustrations of the non-
heredity of acquired characters, and a proof that the effects of
use and disuse, even if inherited, must be overpowered by
natural selection (Chap. XIV) ; and (7) a new argument as to
the nature and origin of the moral and intellectual faculties
of man (Chap. XV).

Although I maintain, and even enforce, my differences
from some of Darwin's views, my whole work tends forcibly
to illustrate the overwhelming importance of Natural Selec-
tion over all other agencies in the production of new species.

PEEFACE

I thus take up Darwin's earlier position, from which he some-
what receded in the later editions of his works, on account
of criticisms and objections which I have endeavoured to show
are unsound. Even in rejecting that phase of sexual selection
depending on female choice, I insist on the greater efficacy
of natural selection. This is pre-eminently the Darwinian
doctrine, and I therefore claim for my book the position of
being the advocate of pure Darwinism.

I wish to express my obligation to Mr. Francis Darwin for
lending me some of his father's unused notes, and to many other
friends for facts or information, which have, I believe, been
acknowledged either in the text or footnotes. Mr. James Sime
has kindly read over the proofs and given me many useful
suggestions ; and I have to thank Professor Meldola, Mr.
Hemsley, and Mr. E. B. Poulton for valuable notes or
corrections in the later chapters in which their special subjects
are touched upon.

Godalming, March 1889.

CONTENTS

CHAPTEE 1

WHAT ARE " SPECIES " AND WHAT IS MEANT BY THEIR
" ORIGIN "

Definition of species — Special creation — The early transmutationists —
Scientific opinion before Darwin — The problem before Darwin — The
change of opinion effected by Darwin — The Darwinian theory — Pro-
posed mode of treatment of the subject . . . Pages 1-13

CHAPTER II

THE STRUGGLE FOR EXISTENCE

Its importance — The struggle among plants — Among animals — Illustrative
cases — Succession of trees in forests of Denmark — The struggle for
existence on the Pampas — Increase of organisms in a geometrical
ratio — Examples of rapid increase of animals — Bapid increase and
wide spread of plants — Great fertility not essential to rapid increase
— Struggle between closely allied species most severe — The ethical
aspect of the struggle for existence . . . 14-40

CHAPTER III

THE VARIABILITY OF SPECIES IN A STATE OF NATURE

Importance of variability— Popular ideas regarding it— Variability of the
lower animals— The variability of insects— Variation among lizards —

CONTEXTS

Variation among birds — -Diagrams of bird -variation — Number of
varying individuals — Variation in the mammalia- — Variation in
internal organs — Variations in the skull — Variations in the habits of
animals — The variability of plants — Species which vary little — Con-
cluding remarks ..... Pages 41-82

CHAPTEE IV

VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED
PLANTS

The facts of variation and artificial selection — Proofs of the generality of
variation — Variations of apples and melons — Variations of flowers —
Variations of domestic animals — Domestic pigeons — Acclimatisation
— Circumstances favourable to selection by man — Conditions favour-
able to variation — Concluding remarks . . . 83-101

CHAPTEE V

NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE
FITTEST

Effect of struggle for existence under unchanged conditions — The effect
under change of conditions — Divergence of character — In insects — In
birds — In mammalia — Divergence leads to a maximum of life in each
area — Closely allied species inhabit distinct areas — Adaptation to
conditions at various periods of life — The continued existence of low
forms of life — Extinction of low types among the higher animals —
Circumstances favourable to the origin of new species — Probable
origin of the dippers — The importance of isolation — On the advance
of organisation by natural selection — Summary of the first five
chapters . . . . . . 102-125

CHAPTEE VI

DIFFICULTIES AND OBJECTIONS

Difficulty as to smallness of variations — As to the right variations occur-
ring when required — The beginnings of important organs — The mam-
mary glands — The eyes of flatfish — Origin of the eye — Useless or
non-adaptive characters — Recent extension of the region of utility in

CONTENTS xi

plants — The same in animals — Uses of tails — Of the horns of deer —
Of the scale-ornamentation of reptiles — Instability of non-adaptive
characters — Delbcauf's law — No "specific" character proved to be
useless — The swamping effects of intercrossing — Isolation as prevent-
ing intercrossing — Gulick on the effects of isolation — Cases in which
isolation is ineffective . . ... . Pages 126-151

CHAPTER VII

ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES
AND THE USUAL STERILITY OF THEIR HYBRID OFFSPRING

Statement of the problem — Extreme susceptibility of the reproductive
functions — Reciprocal crosses — Individual differences in respect to
cross- fertilisation — Dimorphism and trimorphism among plants —
Cases of the fertility of hybrids and of the infertility of mongrels
— The effects of close interbreeding — Mr. Huth's objections — Fertile
hybrids among animals — Fertility of hybrids among plants — Cases of
sterility of mongrels — Parallelism between crossing and change of
conditions — Remarks on the facts of hybridity — Sterility due to
changed conditions and usually correlated with other characters —
Correlation of colour with constitutional peculiarities — The isolation
of varieties by selective association — The influence of natural selection
upon sterility and fertility — Physiological selection — Summary and
concluding remarks ..... 152-186

CHAPTER VIII

THE ORIGIN AND USES OF COLOUR IN ANIMALS

The Darwinian theory threw new light on organic colour — The problem to
be solved — The constancy of animal colour indicates utility — Colour
and environment— Arctic animals white — Exceptions prove the rule —
Desert, forest, nocturnal, and oceanic animals — General theories of
animal colour — Variable protective colouring — Mr. Poulton's experi-
ments— Special or local colour adaptations — Imitation of particular
objects — How they have been produced — Special protective colouring
of butterflies — Protective resemblance among marine animals — Pro-
tection by terrifying enemies — Alluring coloration — The coloration
of birds' eggs — Colour as a means of recognition — Summary of the
preceding exposition — Influence of locality or of climate on colour — ■
Concluding remarks ..... 187-231

CONTENTS

CHAPTER IX

WARNING COLORATION AND MIMICRY

The skunk as an example of warning coloration — Warning colours among
insects — Butterflies — Caterpillars — Mimicry — How mimicry has been
produced — Heliconidae — Perfection of the imitation — Other cases of
mimicry among Lepidoptera — Mimicry among protected groups — Its
explanation — Extension of the principle — Mimicry in other orders of
insects — Mimicry among the vertebrata — Snakes — The rattlesnake and
the cobra — Mimicry among birds — Objections to the theory of mimicry
— Concluding remarks on warning colours and mimicry

Pages 232-267

CHAPTER X

COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX

Sex colours in the mollusca and Crustacea — In insects — In butterflies and
moths — Probable causes of these colours — Sexual selection as a
supposed cause — Sexual coloration of birds — Cause of dull colours of
female birds — Relation of sex colour to nesting habits — Sexual colours
of other vertebrates— Sexual selection by the struggles of males —
Sexual characters due to natural selection — Decorative plumage of
males and its effect on the females — Display of decorative plumage
by the males — A theory of animal coloration — The origin of accessory
plumes — Development of accessory plumes and their display — The
effect of female preference will be neutralised by natural selection —
General laws of animal coloration— Concluding remarks . 268-300

CHAPTER XI

THE SPECIAL COLOURS OF PLANTS : THEIR ORIGIN
AND PURPOSE

The general colour relations of plants — Colours of fruits — -The meaning of
nuts — Edible or attractive fruits — The colours of flowers — -Modes of
securing cross-fertilisation — The interpretation of the facts — Summary

CONTENTS

of additional facts bearing on insect fertilisation — Fertilisation of
flowers by birds— Self-fertilisation of flowers — Difficulties and con-
tradictions— Intercrossing not necessarily advantageous — Supposed
evil results of close interbreeding — How the struggle for existence
acts among flowers — Flowers the product of insect agency — Concluding
remarks on colour in nature .... Pages 301-337

CHAPTER XII

THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS

The facts to be explained — The conditions which have determined dis-
tribution— The permanence of oceans — Oceanic and continental areas
— Madagascar and New Zealand — The teachings of the thousand-
fathom^ line — The distribution of marsupials — The distribution of
tapirs — Powers of dispersal as illustrated by insular organisms — Birds
and insects at sea — Insects at great altitudes — The dispersal of plants
— Dispersal of seeds by the wind — Mineral matter carried by the wind
— Objections to the theory of wind-dispersal answered — Explanation
of north temperate plants in the southern hemisphere — No proof of
glaciation in the tropics — Lower temperature not needed to explain
the facts — Concluding remarks .... 338-374

CHAPTER XIII

THE GEOLOGICAL EVIDENCES OF EVOLUTION

What we may expect — The number of known species of extinct animals —
Causes of the imperfection of the geological record — Geological
evidences of evolution — Shells — Crocodiles — The rhinoceros tribe —
The pedigree of the horse tribe — Development of deer's horns— Brain
development — Local relations of fossil and living animals — Cause
of extinction of large animals — Indications of general progress in
plants and animals— The progressive development of plants— Possible
cause of sudden late appearance of exogens — Geological distribu-
tion of insects — Geological succession of vertebrata — Concluding
remarks .. = •••• 37j-409

CONTENTS

CHAPTER XIV

FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION
AND HEREDITY

Fundamental difficulties and objections — Mr. Herbert Spencer's factors
of organic evolution — Disuse and effects of withdrawal of natural
selection — Supposed effects of disuse among wild animals — Difficulty
as to co-adaptation of parts by variation and selection — Direct action
of the environment— The American school of evolutionists— Origin
of the feet of the ungulates— Supposed action of animal intelligence—
Semper on the direct influence of the environment — Professor Geddes's
theory of variation in plants — Objections to the theory — On the
origin of spines — Variation and selection overpower the effects of use
and disuse— Supposed action of the environment in imitating varia-
tions— Weismann's theory of heredity — The cause of variation — The
non-heredity of acquired characters — The theory of instinct — Con-
cluding remarks ..'... Pages 410-444

CHAPTER XV

DARWINISM APPLIED TO MAN

General identity of human and animal structure — Rudiments and varia-
tions showing relation of man to other mammals — The embryonic
development of man and other mammalia — Diseases common to man
and the lower animals — The animals most nearly allied to man —
The brains of man and apes — External differences of man and apes —
Summary of the animal characteristics of man — The geological
antiquity of man— The probable birthplace of man— The origin of
the moral and intellectual nature of man — The argument from
continuity — The origin of the mathematical faculty — The origin of
the musical and artistic faculties — Independent proof that these
faculties have not been developed by natural selection — The inter-
pretation of the facts — Concluding remarks . . 445-478

INDEX ...-'..... 479-494

LIST OF ILLUSTKATIONS

Portrait of Author . ... Frontispiece

Map showing the 1000-fathom line . . To face page 349

FIG. PAGE

1. Diagram op Yariations op Lacerta mtjralis . . 47

2. ,, Variation of Lizards . . . .48

3. ,, Variation op wings and tail of Birds . 53

4. ,, Variation of Dolichonyx oryzivorus . 55

5. ,, Variation of Agel^ius phceniceus . . 56

6. ,, Variation of Cardinalis virginianus . 58

7. ,, Variation of tarsus and toes . . 60

8. ,, Variation of Birds in Leyden Museum . 61

9. ,, Variation of Icterus Baltimore . . 63

10. „ Variation of Agel^us phceniceus . . 64

11. ,, Curves of Variation . . . .64

12. ,, Variation of Cardinalis virginianus . 65

13. ,, Variation of Sciurus carolinensis . . 67

14. ,, Variation op skulls of Wolf . . 70

15. ,, Variation of skulls of Ursus labiatus . 72

16. „ Variation of skulls of Sus cristatus . 73

17. Primula veris (Cowslip). From Darwin's Forms of Mowers . 157

18. Gazella scemmerringi (to show recognition marks) . . 219

19. Recognition marks of African Plovers (from Seebohm's

CJiaradriadm . ■ ■ • ■ • .221

xvi LIST OF ILLUSTRATIONS

FIG. PAGE

20. Recognition of CEdicnemus veemiculatus and CE. senega-

lensis (from Seebohm's Charadriadce) . . . 223

21. Recognition of Cursorius chalcopterus and C. gallicus

(from Seebohm's Charadriadce) .... 224

22. Recognition of Scolopax megala and S. stenura (from

Seebohm's Charadriadce) ..... 225

23. Methona psidii and Leptalis orise .... 241

24. Opthalmis lincea and Artaxa simulans (from the Official

Narrative of the Voyage of the Challenger) . . . 247

25. Wings of Ituna Ilione and Thyridia megisto (from Pro-

ceedings of the Entomological Society) .... 251

26. Mygnimia aviculus and Coloboehombus fasciatipennis . 259

27. Mimicking Insects from the Philippines (from Semper's

Animal Life) ....... 260

28. Malva sylvestris and M. rotundifolia (from Lubbock's

British Wild Flowers in Relation to Insects) . . . 311

29. Lythrtjm salicaria, three forms of (from Lubbock's British

Wild Flowers in Relation to Insects) .... 312

30. Orchis pyramidalis (from Darwin's Fertilisation of Orchids) . 314

31. Humming-bird fertilising Marcgravia nepenthoides . 320

32. Diagram of mean height of Land and depth of Oceans 345

33. Geological development of the Horse tribe (from Huxley's

American Addresses) ...... 388

34. Diagram illustrating the Geological Distribution of

Plants (from "Ward's Sketch of Palccobotany) . . . 402

35. Transformation of Artemia salina to A. Milhausenii

(from Semper's Animal Life) ..... 426

36. Branchipus stagnalis and Artemia salina (from Semper's

Animal Life) ....... 427

37. Chimpanzee (Troglodytes niger) . , . .454

CHAPTER I

WHAT ARE "SPECIES," AND WHAT IS MEANT BY
THEIR "ORIGIN"

Definition of species — Special creation — The early Transmutationists —
Scientific opinion before Darwin — The problem before Darwin —
The change of opinion effected bjr Darwin — The Darwinian theory
— Proposed mode of treatment of the subject.

The title of Mr. Darwin's great work is — On the Origin of
Species by means of Natural Selection and the Preservation of
Favoured Races in the Struggle for Life. In order to ap-
preciate fully the aim and object of this work, and the
change which it has effected not only in natural history but
in many other sciences, it is necessary to form a clear con-
ception of the meaning of the term " species," to know what
was the general belief regarding them at the time when Mr.
Darwin's book first appeared, and to understand what he
meant, and what was generally meant, by discovering their
" origin." It is for want of this preliminary knowledge that
the majority of educated persons who are not naturalists are
so ready to accept the innumerable objections, criticisms, and
difficulties of its opponents as proofs that the Darwinian
theory is unsound, while it also renders them unable to ap-
preciate, or even to comprehend, the vast change which that
theory has effected in the whole mass of thought and opinion
on the great question of evolution.

The term " species " was thus defined by the celebrated
botanist De Candolle : " A species is a collection of all the
individuals which resemble each other more than they
resemble anything else, which can by mutual fecundation

dB B

DARWINISM

produce fertile individuals, and which reproduce themselves
by generation, in such a manner that we may from analogy
suppose them all to have sprung from one single individual."
And the zoologist Swainson gives a somewhat similar defini-
tion : " A species, in the usual acceptation of the term, is an
animal which, in a state of nature, is distinguished by certain
peculiarities of form, size, colour, or other circumstances, from
another animal. It propagates, 'after its kind,' individuals
perfectly resembling the parent ; its peculiarities, therefore,
are permanent." 1

To illustrate these definitions we will take two common
English birds, the rook (Corvus frugilegus) and the crow
(Corvus corone). These are distinct species, because, in the first
place, they always differ from each other in certain slight
peculiarities of structure, form, and habits, and, in the second
place, because rooks always produce rooks, and crows produce
crows, and they do not interbreed. It was therefore con-
cluded that all the rooks in the world had descended from a
single pair of rooks, and the crows in like manner from a
single pair of crows, while it was considered impossible that
crows could have descended from rooks or vice versa. The
" origin " of the first pair of each kind was a mystery.
Similar remarks may be applied to our two common plants,
the sweet violet (Viola odorata) and the dog violet (Viola
canina). These also produce their like and never produce
each other or intermingle, and they were therefore each
supposed to have sprung from a single individual whose
" origin " was unknown. But besides the crow and the rook
there are about thirty other kinds of birds in various parts of
the world, all so much like our species that they receive the
common name of crows ; and some of them differ less from
each other than does our crow from our rook. These are all
species of the genus Corvus, and were therefore believed to
have been always as distinct as they are now, neither more
nor less, and to have each descended from one pair of ances-
tral crows of the same identical species, which themselves had
an unknown " origin." Of violets there are more than a
hundred different kinds in various parts of the world, all
differing very slightly from each other and forming distinct
1 Geography and Classification oj Animals, p. 350.

WHAT ARE SPECIES

species of the genus Viola. But, as these also each produce
their like and do not intermingle, it was believed that every-
one of them had always been as distinct from all the others as
it is now, that all the individuals of each kind had descended
from one ancestor, but that the " origin " of these hundred
slightly differing ancestors was unknown. In the words of
Sir John Herschel, quoted by Mr. Darwin, the origin of
such species was " the mystery of mysteries."

The Early Transmutationists.

A few great naturalists, struck by the very slight difference
between many of these species, and the numerous links that
exist between the most different forms of animals and plants,
and also observing that a great many species do vary con-
siderably in their forms, colours, and habits, conceived the idea
that they might be all produced one from the other. The
most eminent of these writers was a great French naturalist,
Lamarck, who published an elaborate work, the Philosophic
Zoologique, in which he endeavoured to prove that all ani-
mals whatever are descended from other species of animals.
He attributed the change of species chiefly to the effect of
changes in the conditions of life — such as climate, food, etc. —
and especially to the desires and efforts of the animals them-
selves to improve their condition, leading to a modification of
form or size in certain parts, owing to the well-known physio-
logical law that all organs are strengthened by constant use,
while they are weakened or even completely lost by disuse.
The arguments of Lamarck did not, however, satisfy naturalists,
and though a few adopted the view that closely allied species
had descended from each other, the general belief of the
educated public was, that each species was a " special creation"
quite independent of all others ; while the great body of
naturalists equally held, that the change from one species
to another by any known law or cause was impossible,
and that the " origin of species " was an unsolved and
probably insoluble problem. The only other important work
dealing with the question was the celebrated Vestiges of
Creation, published anonymously, but now acknowledged to
have been written by the late Robert Chambers. In this
work the action of general laws was traced throughout the

DARWINISM

universe as a system of growth and development, and it was
argued that the various species of animals and plants had
been produced in orderly succession from each other by the
action of unknown laws of development aided by the action
of external conditions. Although this work had a consider-
able effect in influencing jDublic opinion as to the extreme
improbability of the doctrine of the independent " special
creation " of each species, it had little effect upon natural-
ists, because it made no attempt to grapple with the problem
in detail, or to show in any single case how the allied species
of a genus could have arisen, and have preserved their
numerous slight and apparently purposeless differences from
each other. No clue whatever was afforded to a law which
should produce from any one species one or more slightly
differing but yet permanently distinct species, nor was any
reason given why such slight yet constant differences should
exist at all.

Scientific Opinion before Darwin.

In order to shoAv how little effect these writers had upon
the public mind, I will quote a few passages from the
writings of Sir Charles Lyell, as representing the opinions
of the most advanced thinkers in the period immediately
preceding that of Darwin's work. When recapitulating the
facts and arguments in favour of the invariability and
permanence of species, he says : " The entire variation from
the original type which any given kind of change can pro-
duce may usually be effected in a brief period of time, after
which no further deviation can be obtained by continuing to
alter the circumstances, though ever so gradually, indefinite
divergence either in the way of improvement or deterioration
being prevented, and the least possible excess beyond the
defined limits beimr fatal to the existence of the individual."
In another place he maintains that " varieties of some species
may differ more than other species do from each other
without shaking our confidence in the reality of species."
He further adduces certain facts in geology as being, in his
opinion, "fatal to the theory of progressive development,"
and he explains the fact that there are so often distinct
species in countries of similar climate and vegetation by

WHAT ARE SPECIES

" special creations " in each country ; and these conclusions
were arrived at after a careful study of Lamarck's work, a full
abstract of Avhich is given in the earlier editions of the
Principles of Geology.1

Professor Agassiz, one of the greatest naturalists of the last
generation, went even further, and maintained not only that
each species was specially created, but that it was created in
the proportions and in the localities in which we now find it
to exist. The following extract from his very instructive
book on Lake Superior explains this view: "There are in
animals peculiar adaptations which are characteristic of their
species, and which cannot be supposed to have arisen from
subordinate influences. Those which live in shoals cannot be
supposed to have been created in single pairs. Those which
are made to be the food of others cannot have been created
in the same proportions as those which live upon them.
Those which are everywhere found in innumerable specimens
must have been introduced in numbers capable of maintaining
their normal proportions to those which live isolated and are
comparatively and constantly fewer. For we know that this
harmony in the numerical proportions between animals is
one of the great laws of nature. The circumstance that
species occur within definite limits where no obstacles prevent
their wider distribution leads to the further inference that
these limits were assigned to them from the beginning, and
so we should come to the final conclusion that the order
which prevails throughout nature is intentional, that it is
regulated by the limits marked out on the first day of
creation, and that it has been maintained unchanged through
ages with no other modifications than those which the higher
intellectual powers of man enable him to impose on some
few animals more closely connected with him." 2

These opinions of some of the most eminent and influential
writers of the pre -Darwinian age seem to us, now, either
altogether obsolete or positively absurd ; but they never-
theless exhibit the mental condition of even the most
advanced section of scientific men on the problem of the

1 These expressions occur in Chapter IX. of the earlier editions (to the
ninth) of the Principles of Geology.
- L. Agassiz, Lake Superior, p. 377.

DARWINISM

nature and origin of species. They render it clear that,
notwithstanding the vast knowledge and ingenious reasoning
of Lamarck, and the more general exposition of the subject by
the author of the Vestiges of Creation, the first step had not
been taken towards a satisfactory explanation of the deriva-
tion of any one species from any other. Such eminent
naturalists as Geoffroy Saint Hilaire, Dean Herbert, Professor
Grant, Von Buch, and some others, had expressed their belief
that species arose as simple varieties, and that the species of
each genus were all descended from a common ancestor ; but
none of them gave a clue as to the law or the method by
which the change had been effected. This was still " the great
mystery." As to the further question — how far this common
descent could be carried ; whether distinct families, such as
crows and thrushes, could possibly have descended from each
other; or, whether all birds, including such widely distinct
types as wrens, eagles, ostriches, and ducks, could all be the
modified descendants of a common ancestor ; or, still further,
whether mammalia, birds, reptiles, and fishes, could all have
had a common origin ; — these questions had hardly come up
for discussion at all, for it was felt that, while the very first
step along the road of " transmutation of species " (as it was
then called) had not been made, it was quite useless to
speculate as to how far it might be possible to travel in the
same direction, or where the road would ultimately lead to.

The Problem before Darwin.

It is clear, then, that what was understood by the " origin "
or the " transmutation " of species before Darwin's work
appeared, was the comparatively simple question whether the
allied species of each genus had or had not been derived from
one another and, remotely, from some common ancestor, by
the ordinary method of reproduction and by means of laws
and conditions still in action and capable of being thoroughly
investigated. If any naturalist had been asked at that day
Avhether, supposing it to be clearly shown that all the different
species of each genus had been derived from some one
ancestral species, and that a full and complete explanation
were to be given of how each minute difference in form,
colour, or structure might have originated, and how the

WHAT ARE SPECIES

several peculiarities of habit and of geographical distribution
might have been brought about — whether, if this were done,
the "origin of species " would be discovered, the great
mystery solved, he would undoubtedly have replied in the
affirmative. He would probably have added that he never
expected any such marvellous discovery to be made in
his lifetime. But so much as this assuredly Mr. Darwin has
done, not only in the opinion of his disciples and admirers,
but by the admissions of those who doubt the completeness
of his explanations. For almost all their objections and
difficulties apply to those larger differences which separate
genera, families, and orders from each other, not to those which
separate one species from the species to which it is most nearly
allied, and from the remaining species of the same genus. They
adduce such difficulties as the first development of the eye, or
of the milk-producing glands of the mammalia ; the wonderful
instincts-of bees and of ants ; the complex arrangements for
the fertilisation of orchids, and numerous other points of
structure or habit, as not being satisfactorily explained. But
it is evident that these peculiarities had their origin at a very
remote period of the earth's history, and no theory, however
complete, can do more than afford a probable conjecture as to
how they were produced. Our ignorance of the state of the
earth's surface and of the conditions of life at those remote
periods is very great ; thousands of animals and plants must
have existed of which we have no record ; while we are
usually without any information as to the habits and general
life-history even of those of which we possess some fragmentary
remains ; so that the truest and most complete theory would
not enable us to solve all the difficult problems which the
whole course of the development of life upon our globe
presents to us.

What we may expect a true theory to do is to enable us
to comprehend and follow out in some detail those changes in
the form, structure, and relations of animals and plants which
are effected in short periods of time, geologically speaking,
and which are now going on around us. We may expect it
to explain satisfactorily most of the lesser and superficial
differences which distinguish one species from another. We
may expect it to throw light on the mutual relations of the

DARWINISM

animals and plants which live together in any one country,
and to give some rational account of the phenomena presented
by their distribution in different parts of the world. And,
lastly, we may expect it to explain many difficulties and to
harmonise many incongruities in the excessively complex
affinities and relations of living things. All this the Darwinian
theory undoubtedly does. It shows us how, by means of
some of the most universal and ever-acting laws in nature,
new species are necessarily produced, while the old species
become extinct ; and it enables us to understand how the
continuous action of these laws during the long periods with
which geology makes us acquainted is calculated to bring
about those greater differences presented by the distinct
genera, families, and orders into which all living things are
classified by naturalists. The differences which these present
are all of the same nature as those presented by the species of
many large genera, but much greater in amount ; and they can
all be explained by the action of the same general laws and
by the extinction of a larger or smaller number of intermediate
species. Whether the distinctions between the higher groups
termed Classes and Sub-kingdoms may be accounted for in
the same way is a much more difficult question. The differ-
ences which separate the mammals, birds, reptiles, and fishes
from each other, though vast, yet seem of the same nature as
those which distinguish a mouse from an elephant or a
swallow from a goose. But the vertebrate animals, the
mollusca, and the insects, are so radically distinct in their
whole organisation and in the very plan of their structure,
that objectors may not unreasonably doubt whether they can
all have been derived from a common ancestor by means of
the very same laws as have sufficed for the differentiation
of the various species of birds or of reptiles.

The Change of Opinion- effected by Darwin.

The point I wish especially to urge is this. Before
Darwin's work appeared, the great majority of naturalists, and
almost without exception the whole literary and scientific
world, held firmly to the belief that species were realities, and
had not been derived from other species by any process
accessible to us ; the different species of crow and of violet

WHAT ARE SPECIES

were believed to have been always as distinct and separate as
they are now, and to have originated by some totally unknown
process so far removed from ordinary reproduction that it was
usually spoken of as " special creation." There was, then, no
question of the origin of families, orders, and classes, because
the very first step of all, the " origin of species," was believed
to be an insoluble problem. But now this is all changed. The
whole scientific and literary world, even the whole educated
public, accepts, as a matter of common knowledge, the origin
of species from other allied species by the ordinary process of
natural birth. The idea of special creation or any altogether
exceptional mode of production is absolutely extinct ! Yet
more : this is held also to apply to many higher groups as
well as to the species of a genus, and not even Mr. Darwin's
severest critics venture to suggest that the primeval bird,
reptile, or fish must have been " specially created." And this
vast, this'totally unprecedented change in public opinion has
been the result of the work of one man, and was brought
about in the short space of twenty years ! This is the answer
to those who continue to maintain that the "origin of species" is
not yet discovered ; that there are still doubts and difficulties ;
that there are divergencies of structure so great that we
cannot understand how they had their beginning. We may
admit all this, just as we may admit that there are enormous
difficulties in the way of a complete comprehension of the
origin and nature of all the parts of the solar system and of
the stellar universe. But we claim for Darwin that he is the
Newton of natural history, and that, just so surely as that the
discovery and demonstration by Newton of the law of gravita-
tion established order in place of chaos and laid a sure founda-
tion for all future study of the starry heavens, so surely has
Darwin, by his discovery of the law of natural selection
and his demonstration of the great principle of the preserva-
tion of useful variations in the struggle for life, not only thrown
a flood of light on the process of development of the whole
organic world, but also established a firm foundation for all
future study of nature.

In order to show the view Darwin took of his own work,
and what it was that he alone claimed to have done, the
concluding passage of the introduction to the Origin of

10 DARWINISM chap.

Species should be carefully considered. It is as follows :
"Although much remains obscure, and will long remain
obscure, I can entertain no doubt, after the most deliberate
and dispassionate judgment of which I am capable, that the
view which most naturalists until recently entertained and
which I formerly- entertained — namely, that each species has
been independently created — is erroneous. I am fully con-
vinced that species are not immutable ; but that those
belonging to what are called the same genera are lineal
descendants of some other and generally extinct species, in
the same manner as the acknowledged varieties of any one
species are the descendants of that species. Furthermore, I
am convinced that Natural Selection has been the most im-
portant, but not the exclusive, means of modification."

It should be especially noted that all which is here claimed
is now almost universally admitted, while the criticisms of
Darwin's works refer almost exclusively to those numerous
questions which, as he himself says, " will long remain
obscure."

The Darwinian Theory.

As it will be necessary, in the following chapters, to set
forth a considerable body of facts in almost every department
of natural history, in order to establish the fundamental
propositions on which the theory of natural selection rests,
I propose to give a preliminary statement of what the theory
really is, in order that the reader • may better appreciate the
necessity for discussing so many details, and may thus feel a
more enlightened interest in them. Many of the facts to be
adduced are so novel and so curious that they are sure to be
appreciated by every one who takes an interest in nature, but
unless the need of them is clearly seen it may be thought that
time is being wasted on mere curious details and strange facts
which have little bearing on the question at issue.

The theory of natural selection rests on two main classes
of facts which apply to all organised beings without exception,
and which thus take rank as fundamental principles or laws.
The first is, the power of rapid multiplication in a geometrical
progression ; the second, that the offspring always vary slightly
from the parents, though generally very closely resembling

WHAT ARE SPECIES 11

them. From the first fact or law there follows, necessarily, a
constant struggle for existence ; because, while the offspring
always exceed the parents in number, generally to an enormous
extent, yet the total number of living organisms in the world
does not, and cannot, increase year by year. Consequently
every year, on the average, as many die as are born, plants as
well as animals ; and the majority die premature deaths.
They kill each other in a thousand different ways ; they starve
each other by some consuming the food that others want ;
they are destroyed largely by the powers of nature — by cold
and heat, by rain and storm, by flood and fire. There is thus
a perpetual struggle among them which shall live and which
shall die ; and this struggle is tremendously severe, because
so few can possibly remain alive — one in five, one in ten, often
only one in a hundred or even one in a thousand.

Then comes the question, Why do some live rather than
others 1 If all the individuals of each species were exactly
alike in every respect, we could only say it is a matter of
chance. But they are not alike. We find that they vary in
many different ways. Some are stronger, some swifter, some
hardier in constitution, some more cunning. An obscure
colour may render concealment more easy for some, keener
sight may enable others to discover prey or escape from an
enemy better than their fellows. Among plants the smallest
differences may be useful or the reverse. The earliest and
strongest shoots may escape the slug ; their greater vigour
may enable them to flower and seed earlier in a wet autumn ;
plants best armed with spines or hairs may escape being
devoured ; those whose flowers are most conspicuous may be
soonest fertilised by insects. We cannot doubt that, on the
whole, any beneficial variations will give the possessors of it a
greater probability of living through the tremendous ordeal
they have to undergo. There may be something left to
chance, but on the whole the fittest will survive.

Then we have another important fact to consider, the
principle of heredity or transmission of variations. If we
grow plants from seed or breed any kind of animals year
after year, consuming or giving away all the increase we do
not wish to keep just as they come to hand, our plants or
animals will continue much the same ; but if every year we

12 DARWINISM

carefully save the best seed to sow and the finest or brightest
coloured animals to breed from, we shall soon find that an
improvement will take place, and that the average quality of
our stock will be raised. This is the way in which all our
fine garden fruits and vegetables and flowers have been pro-
duced, as well as -all our splendid breeds of domestic animals ;
and they have thus become in many cases so different from
the wild races from which they originally sprang as to be
hardly recognisable as the same. It is therefore proved that
if any particular kind of variation is preserved and bred from,
the variation itself goes on increasing in amount to an
enormous extent ; and the bearing of this on the question of
the origin of species is most important. For if in each
generation of a given animal or plant the fittest survive to
continue the breed, then whatever may be the special
peculiarity that causes "fitness" in the particular case, that
peculiarity will go on increasing and strengthening so long as
it is useful to the species. But the moment it has reached its
maximum of usefulness, and some other quality or modifica-
tion would help in the struggle, then the individuals which
vary in the new direction will survive ; and thus a species may
be gradually modified, first in one direction, then in another,
till it differs from the original parent form as much as the
greyhound differs from any wild dog or the cauliflower from
any wild plant. But animals or plants which thus differ in
a state of nature are always classed as distinct species, and
thus we see how, by the continuous survival of the fittest
or the preservation of favoured races in the struggle for life,
new species may be originated.

This self-acting process which, by means of a few easily
demonstrated groups of facts, brings about change in the
organic world, and keeps each species in harmony with the
conditions of its existence, will appear to some persons so
clear and simple as to need no further demonstration. But
to the great majority of naturalists and men of science endless
difficulties and objections arise, owing to the wonderful variety
of animal and vegetable forms, and the intricate relations of
the different species and groups of species with each other;
and it was to answer as many of these objections as possible,
and to show that the more we know of nature the more we

WHAT ARE SPECIES 13

find it to harmonise with the development hypothesis, that
Darwin devoted the whole of his life to collecting facts and
making experiments, the record of a portion of which he has
given us in a series of twelve masterly volumes.

Proposed Mode of Treatment of the Subject.

It is evidently of the most vital importance to any theory
that its foundations should be absolutely secure. It is
therefore necessary to show, by a wide and comprehensive
array of facts, that animals and plants do perpetually vary in
the manner and to the amount requisite ; and that this takes
place in wild animals as well as in those which are domesti-
cated. It is necessary also to prove that all organisms do
tend to increase at the great rate alleged, and that this
increase actually occurs, under favourable conditions. We
have to prove, further, that variations of all kinds can be
increasechand accumulated by selection ; and that the struggle
for existence to the extent here indicated actually occurs in
nature, and leads to the continued preservation of favourable
variations.

These matters will be discussed in the four succeeding
chapters, though in a somewhat different order — the struggle
for existence and the power of rapid multiplication, which is
its cause, occupying the first place, as comprising those facts
which are the most fundamental and those which can be
perfectly explained without any reference to the less generally
understood facts of variation. These chapters will be followed
by a discussion of certain difficulties, and of the vexed question
of hybridity. Then will come a rather full account of the
more important of the complex relations of organisms to each
other and to the earth itself, which are either fully explained
or greatly elucidated by the theory. The concluding chapter
will treat of the origin of man and his relations to the lower
animals.

CHAPTER II

THE STRUGGLE FOR EXISTENCE

Its importance — The struggle among plants — Among animals — Illustrative
cases — Succession of trees in forests of Denmark- — The struggle for
existence on the Pampas — Increase of organisms in a geometrical
ratio — Examples of great powers of increase of animals — Rapid
increase and wide spread of plants — Great fertility not essential to
rapid increase — Struggle between closely allied species most severe —
The ethical aspect of the struggle for existence.

There is perhaps no phenomenon of nature that is at once
so important, so universal, and so little understood, as the
struggle for existence continually going on among all organ-
ised beings. To most persons nature a/ppears calm, orderly,
and peaceful. They see the birds singing in the trees, the
insects hovering over the flowers, the squirrel climbing among
the tree-tops, and all living things in the possession of health
and vigour, and in the enjoyment of a sunny existence. But
they do not see, and hardly ever think of, the means by which
this beauty and harmony and enjoyment is brought about.
They do not see the constant and daily search after food, the
failure to obtain which means weakness or death ; the con-
stant effort to escape enemies ; the ever -recurring struggle
against the forces of nature. This daily and hourly struggle,
this incessant warfare, is nevertheless the very means by which
much of the beauty and harmony and enjoyment in nature is
produced, and also affords one of the most important elements
in bringing about the origin of species. We must, therefore,
devote some time to the consideration of its various aspects
and of the many curious phenomena to which it gives rise.

It is a matter of common observation that if weeds are
allowed to grow unchecked in a garden they will soon destroy

chap, n THE STRUGGLE FOR EXISTENCE 15

a number of the flowers. It is not so commonly known that
if a garden is left to become altogether wild, the weeds that
first take possession of it, often covering the whole surface of
the ground with two or three different kinds, will themselves
be supplanted by others, so that in a few years many of
the original flowers and of the earliest weeds may alike have
disappeared. This is one of the very simplest cases of the
struggle for existence, resulting in the successive displacement
of one set of species by another ; but the exact causes of this
displacement are by no means of such a simple nature. All
the plants concerned may be perfectly hardy, all may grow
freely from seed, yet when left alone for a number of years,
each set is in turn driven out by a succeeding set, till at the
end of a considerable period — a century or a few centuries
perhaps — hardly one of the plants which first monopolised
the ground would be found there.

Another phenomenon of an analogous kind is presented by
the different behaviour of introduced wild plants or animals
into countries apparently quite as well suited to them as
those which they naturally inhabit. Agassiz, in his work on
Lake Superior, states that the roadside weeds of the north-
eastern United States, to the number of 130 species, are all
European, the native weeds having disappeared westwards ;
and in New Zealand there are no less than 250 species of
naturalised European plants, more than 100 species of which
have spread widely over the country, often displacing the
native vegetation. On the other hand, of the many hundreds
of hardy plants which produce seed freely in our gardens,
very few ever run wild, and hardly any have become common.
Even attempts to naturalise suitable plants usually fail ; for
A. de Candolle states that several botanists of Paris, Geneva,
and especially of Montpellier, have sown the seeds of many
hundreds of species of hardy exotic plants in what appeared
to be the most favourable situations, but that, in hardly a
single case, has any one of them become naturalised.1 Even
a plant like the potato — so widely cultivated, so hardy, and so
well adapted to spread by means of its many-eyed tubers — has
not established itself in a wild state in any part of Europe.
It would be thought that Australian plants would easily run
1 Geographie Botanique, p. 798.

16 DARWINISM

wild in New Zealand. But Sir Joseph Hooker informs us
that the late Mr. Bidwell habitually scattered Australian seeds
during his extensive travels in New Zealand, yet only two or
three Australian plants appear to have established themselves
in that country, and these only in cultivated or newly moved
soil.

These few illustrations sufficiently show that all the plants
of a country are, as De Candolle says, at war with each other,
each one struggling to occupy ground at the expense of its
neighbour. But, besides this direct competition, there is one
not less powerful arising from the exposure of almost all plants
to destruction by animals. The buds are destroyed by birds,
the leaves by caterpillars, the seeds by weevils ; some insects
bore into the trunk, others burrow in the twigs and leaves ;
slugs devour the young seedlings and the tender shoots, wire-
worms gnaw the roots. Herbivorous mammals devour many
species bodily, while some uproot and devour the buried
tubers.

In animals, it is the eggs or the very young that suffer most
from their various enemies ; in plants, the tender seedlings
when they first appear above the ground. To illustrate this
latter point Mr. Darwin cleared and dug a piece of ground
three feet long and two feet wide, and then marked all the
seedlings of weeds and other plants which came up, noting
what became of them. The total number was 357, and out
of these no less than 295 were destroyed by slugs and insects.
The direct strife of plant with plant is almost equally fatal
when the stronger are allowed to smother the weaker. When
turf is mown or closely browsed by animals, a number of
strong and weak plants live together, because none are allowed
to grow much beyond the rest ; but Mr. Darwin found that
when the plants which compose such turf are allowed to
grow up freely, the stronger kill the weaker. In a plot of
turf three feet by four, twenty distinct species of plants were
found to be growing, and no less than nine of these perished
altogether when the other species were allowed to grow up
to their full size.1

But besides having to protect themselves against competing
plants and against destructive animals, there is a yet deadlier
1 The Origin of Species, p. 53.

ii THE STRUGGLE FOR EXISTENCE 17

enemy in the forces of inorganic nature. Each species can
sustain a certain amount of heat and cold, each requires a
certain amount of moisture at the right season, each wants
a proper amount of light or of direct sunshine, each needs
certain elements in the soil ; the failure of a due proportion
in these inorganic conditions causes weakness, and thus leads
to speedy death. The struggle for existence in plants is,
therefore, threefold in character and infinite in complexity,
and the result is seen in their curiously irregular distribution
over the face of the earth. Not only has each country its
distinct plants, but every valley, every hillside, almost every
hedgerow, has a different set of plants from its adjacent valley,
hillside, or hedgerow — if not always different in the actual
species yet very different in comparative abundance, some
which are rare in the one being common in the other. Hence
it happens that slight changes of conditions often produce
great changes in the flora of a country. Thus in 1740 and
the two following years the larva of a moth (Phalsena graminis)
committed such destruction in many of the meadows of
Sweden that the grass was greatly diminished in quantity,
and many plants which were before choked by the grass
sprang up, and the ground became variegated with a multi-
tude of different species of flowers. The introduction of goats
into the island of St. Helena led to the entire destruction of
the native forests, consisting of about a hundred distinct species
of trees and shrubs, the young plants being devoured by
the goats as fast as they grew up. The camel is a still greater
enemy to woody vegetation than the goat, and Mr. Marsh
believes that forests would soon cover considerable tracts of
the Arabian and African deserts if the goat and the camel
were removed from them.1 Even in many parts of our own
country the existence of trees is dependent on the absence of
cattle. Mr. Darwin observed, on some extensive heaths near
Farnham, in Surrey, a few clumps of old Scotch firs, but no
young trees over hundreds of acres. Some portions of the heath
had, however, been enclosed a few years before, and these en-
closures were crowded with young fir-trees growing too close
together for all to live ; and these were not sown or planted,
nothing having, been done to the ground beyond enclosing it
1 The Earth as Modified by Human Action, p. 51.
C

DARWINISM

so as to keep out cattle. On ascertaining this, Mr. Darwin
was so much surprised that he searched among the heather in
the unenclosed parts, and there he found multitudes of little
trees and seedlings which had been perpetually browsed down
by the cattle. In one square yard, at a point about a hundred
yards from one of the old clumps of firs, he counted thirty-
two little trees, and one of them had twenty-six rings of
growth, showing that it had for many years tried to raise its
head above the stems of the heather and had failed. Yet
this heath was very extensive and very barren, and, as Mr.
Darwin remarks, no one would ever have imagined that cattle
would have so closely and so effectually searched it for food.

In the case of animals, the competition and struggle are
more obvious. The vegetation of a given district can only
support a certain number of animals, and the different kinds
of plant-eaters will compete together for it. They will also
have insects for their competitors, and these insects will be
kept down by birds, which will thus assist the mammalia.
But there will also be carnivora destroying the herbivora ;
while small rodents, like the lemming and some of the field-
mice, often destroy so much vegetation as materially to affect
the food of all the other groups of animals. Droughts, floods,
severe winters, storms and hurricanes will injure these in
various degrees, but no one species can be diminished in
numbers without the effect being felt in various complex ways
by all the rest. A few illustrations of this reciprocal action
must be given.

Illustrative Cases of the Struggle for Life.

Sir Charles Lyell observes that if, by the attacks of seals
or other marine foes, salmon are reduced in numbers, the
consequence will be that otters, living far inland, will be
deprived of food and will then destroy many young birds or
quadrupeds, so that the increase of a marine animal may
cause the destruction of many land animals hundreds of miles
away. Mr. Darwin carefully observed the effects produced
by planting a few hundred acres of Scotch fir, in Staffordshire,
on part of a very extensive heath which had never been
cultivated. After the planted portion was about twenty-five
years old he observed that the change in the native vegetation

ii THE STRUGGLE FOR EXISTENCE 19

was greater than is often seen in passing from one quite
different soil to another. Besides a great change in the pro-
portional numbers of the native heath-plants, twelve species
which could not be found on the heath flourished in the
plantations. The effect on the insect life must have been still
greater, for six insectivorous birds which were very common
in the plantations were not to be seen on the heath, which
was, however, frequented by two or three different species of
insectivorous birds. It would have required continued study
for several years to determine all the differences in the
organic life of the two areas, but the facts stated by Mr.
Darwin are sufficient to show how great a change may be
effected by the introduction of a single kind of tree and the
keeping out of cattle.

The next case I will give in Mr. Darwin's own words :
" In several parts of the world insects determine the existence
of cattle.' Perhaps Paraguay offers the most curious instance
of this ; for here neither cattle nor horses nor dogs have ever
run wild, though they swarm southward and northward in a
feral state ; and Azara and Rengger have shown that this is
caused by the greater numbers, in Paraguay, of a certain fly
which lays its eggs in the navels of these animals when first
born. The increase of these flies, numerous as they are,
must be habitually checked by some means, probably by other
parasitic insects. Hence, if certain insectivorous birds were
to decrease in Paraguay, the parasitic insects would probably
increase ; and this would lessen the number of the navel-
frequenting flies — then cattle and horses would become feral,
and this would greatly alter (as indeed I have observed in
parts of South America) the vegetation : this again would
largely affect the insects, and this, as we have just seen in
Staffordshire, the insectivorous birds, and so onward in ever-
increasing circles of complexity. Not that under nature the
relations will ever be as simple as this. Battle within battle
must be continually recurring with varying success; and yet in
the long run the forces are so nicely balanced, that the face
of nature remains for a long time uniform, though assuredly
the merest trifle would give the victory to one organic being
over another."1

1 The Origin of Species, p. 56.

20 DARWINISM

Such cases as the above may perhaps be thought excep-
tional, but there is good reason to believe that they are by no
means rare, but are illustrations of what is going on in every
part of the world, only it is very difficult for us to trace out
the complex reactions that are everywhere occurring. The
general impression of the ordinary observer seems to be that
wild animals and plants live peaceful lives and have
few troubles, each being exactly suited to its place and
surroundings, and therefore having no difficulty in maintain-
'ing itself. Before showing that this view is, everywhere
and always, demonstrably untrue, we will consider one other
case of the complex relations of distinct organisms adduced
by Mr. Darwin, and often quoted for its striking and almost
eccentric character. It is now well known that many flowers
require to be fertilised by insects in order to produce seed,
and this fertilisation can, in some cases, only be effected by
one particular species of insect to which the flower has become
specially adapted. Two of our common plants, the wild heart's-
ease (Viola tricolor) and the red clover (Trifolium pratense), are
thus fertilised by humble-bees almost exclusively, and if these
insects are prevented from visiting the flowers, they produce
either no seed at all or exceedingly few. Now it is known that
field-mice destroy the combs and nests of humble-bees, and
Colonel Newman, who has paid great attention to these insects,
believes that more than two-thirds of all the humble-bees'
nests in England are thus destroyed. But the number of
mice depends a good deal on the number of cats ; and the same
observer says that near villages and towns he has found the
nests of humble-bees more numerous than elsewhere, which he
attributes to the number of cats that destroy the mice.
Hence it follows, that the abundance of red clover and wild
heart's-ease in a district will depend on a good supply of cats
to kill the mice, which would otherwise destroy and keep down
the humble-bees and prevent them from fertilising the flowers.
A chain of connection has thus been found between such
totally distinct organisms as flesh-eating mammalia and sweet-
smelling flowers, the abundance or scarcity of the one closely
corresponding to that of the other !

The following account of the struggle between trees in the
forests of Denmark, from the researches of M. Hansten-

ii THE STRUGGLE FOR EXISTENCE 21

Blangsted, strikingly illustrates our subject.1 The chief com-
batants are the beech and the birch, the former being every-
where successful in its invasions. Forests composed wholly
of birch are now- only found in sterile, sandy tracts; every-
where else the trees are mixed, and wherever the soil is
favourable the beech rapidly drives out the birch. The latter
loses its branches at the touch of the beech, and devotes all
its strength to the upper part where it towers above the beech.
It may live long in this way, but it succumbs ultimately in
the fight — of old age if of nothing else, for the life of the
birch in Denmark is shorter than that of the beech. The
writer believes that light (or rather shade) is the cause of the
superiority of the latter, for it has a greater development of
its branches than the birch, which is more open and thus
allows the rays of the sun to pass through to the soil below,
while the tufted, bushy top of the beech preserves a deep
shade at its base. Hardly any young plants can grow under
the beech except its own shoots ; and while the beech can
flourish under the shade of the birch, the latter dies im-
mediately under the beech. The birch has only been saved
from total extermination by the facts that it had possession of
the Danish forests long before the beech ever reached the
country, and that certain districts are unfavourable to the
growth of the latter. But wherever the soil has been enriched
by the decomposition of the leaves of the birch the battle
begins. The birch still flourishes on the borders of lakes and
other marshy places, where its enemy cannot exist. In the
same way, in the forests of Zeeland, the fir forests are dis-
appearing before the beech. Left to themselves, the firs are
soon displaced by the beech. The struggle between the latter
and the oak is longer and more stubborn, for the branches and
foliage of the oak are thicker, and offer much resistance to the
passage of light. The oak, also, has greater longevity ; but,
sooner or later, it too succumbs, because it cannot develop
in the shadow of the beech. The earliest forests of Denmark
were mainly composed of aspens, with which the birch was
apparently associated ; gradually the soil was raised, and the
climate grew milder ; then the fir came and formed large
forests. This tree ruled for centuries, and then ceded the
1 See Nature, vol. xxxi. p. 63.

22 DARWINISM

first place to the holm-oak, which is now giving way to the
beech. Aspen, birch, fir, oak, and beech appear to be the
steps in the struggle for the survival of the fittest among the
forest-trees of Denmark.

It may be added that in the time of the Romans the
beech was the principal forest-tree of Denmark as it is now,
while in the much earlier bronze age, represented by the later
remains found in the peat bogs, there were no beech-trees, or
very few, the oak being the prevailing tree, while in the still
earlier stone period the fir was the most abundant. The
beech is a tree essentially of the temperate zone, having its
northern limit considerably southward of the oak, fir, birch,
or aspen, and its entrance into Denmark was no doubt due to
the amelioration of the climate after the glacial epoch had
entirely passed away. We thus see how changes of climate,
which are continually occurring owing either to cosmical or
geographical causes, may initiate a struggle among plants
which -may continue for thousands of years, and which must
profoundly modify the relations of the animal world, since
the very existence of innumerable insects, and even of many
birds and mammals, is dependent more or less completely on
certain species of plants.

The Struggle for Existence on the Pampas.

Another illustration of the struggle for existence, in which
both plants and animals are implicated, is afforded by the
pampas of the southern part of South America. The absence
of trees from these vast plains has been imputed by Mr.
Darwin to the supposed inability of the tropical and sub-
tropical forms of South America to thrive on them, and there
being no other source from which they could obtain a supply ;
and that explanation was adopted by such eminent botanists
as Mr. Ball and Professor Asa Gray. This explanation has
always seemed to me unsatisfactory, because there are ample
forests both in the temperate regions of the Andes and on the
whole west coast down to Terra del Fuego; and it is inconsistent
with what we know of the rapid variation and adaptation of
species to new conditions. What seems a more satisfactory
explanation has been given by Mr. Edwin Clark, a civil
engineer, who resided nearly two years in the country and

ii THE STRUGGLE FOR EXISTENCE 23

paid much attention to its natural history. He says : " The
peculiar characteristics of these vast level plains which descend
from the Andes to the great river basin in unbroken monotony,
are the absence of rivers or water-storage, and the periodical
occurrence of droughts, or ' siccos/ in the summer months.
These conditions determine the singular character both of its
flora and fauna.

"The soil is naturally fertile and favourable for the growth
of trees, and they grow luxuriantly wherever they are pro-
tected. The eucalyptus is covering large tracts wherever it
is enclosed, and willows, poplars, and the fig surround every
estancia when fenced in.

"The open plains are covered with droves of horses
and cattle, and overrun by numberless wild rodents, the
original tenants of the pampas. During the long periods
of drought, which are so great a scourge to the country, these
animals are starved by thousands, destroying, in their efforts to
live, every vestige of vegetation. In one of these ' siccos,' at
the time of my visit, no less than 50,000 head of oxen and
sheep and horses perished from starvation and thirst, after tearing
deep out of the soil every trace of vegetation, including the
wiry roots of the pampas-grass. Under such circumstances
the existence of an unprotected tree is impossible. The only
plants that hold their own, in addition to the indestructible
thistles, grasses, and clover, are a little herbaceous oxalis, pro-
ducing viviparous buds of extraordinary vitality, a few poisonous
species, such as the hemlock, and a few tough, thorny dwarf-
acacias and wiry rushes, which even a starving rat refuses.

" Although the cattle are a modern introduction, the
numberless indigenous rodents must always have effectually
prevented the introduction of any other species of plants;
large tracts are still honeycombed by the ubiquitous biscacho,
a gigantic rabbit ; and numerous other rodents still exist, in-
cluding rats and mice, pampas-hares, and the great nutria and
carpincho (capybara) on the river banks."1

Mr. Clark further remarks on the desperate struggle for

existence which characterises the bordering fertile zones,

where rivers and marshy plains permit a more luxuriant and

varied vegetable and animal life. After describing how the

1 A Visit to South America, 1878 ; also Nature, vol. xxxi. pp. 263-339.

U DARWINISM

river sometimes rose 30 feet in eight hours, doing immense
destruction, and the abundance of the larger carnivora and
large reptiles on its banks, he goes on : " But it was among
the flora that the principle of natural selection was most
prominently displayed. In such a district — overrun with
rodents and escaped cattle, subject to floods that carried away
whole islands of botany, and especially to droughts that dried
up the lakes and almost the river itself — no ordinary plant
could live, even on this rich and watered alluvial debris. The
only plants that escaped the cattle were such as were either
poisonous, or thorny, or resinous, or indestructibly tough.
Hence we had only a great development of solanums, talas,
acacias, euphorbias, and laurels. The buttercup is replaced by
the little poisonous yellow oxalis with its viviparous buds ; the
passion-flowers, asclepiads, bignonias, convolvuluses, and climb-
ing leguminous plants escape both floods and cattle by climb-
ing the highest trees and towering overhead in a flood of
bloom. The ground plants are the portulacas, turneras, and
Oenotheras, bitter and ephemeral, on the bare rock, and almost
independent of any other moisture than the heavy dews.
The pontederias, alismas, and plantago, with grasses and
sedges, derive protection from the deep and brilliant pools ;
and though at first sight the ' monte ' doubtless impresses the
traveller as a scene of the wildest confusion and ruin, yet, on
closer examination, we found it far more remarkable as a
manifestation of harmony and law, and a striking example of
the marvellous power which plants, like animals, possess, of
adapting themselves to the local peculiarities of their habitat,
whether in the fertile shades of the luxuriant ' monte ' or on
the arid, parched-up plains of the treeless pampas."

A curious example of the struggle between plants has
been communicated to me by Mr. John Ennis, a resident in
New Zealand. The English water-cress grows so luxuriantly
in that country as to completely choke up the rivers,
sometimes leading to disastrous floods, and necessitating great
outlay to keep the stream open. But a natural remedy has
now been found in planting willows on the banks. The
roots of these trees penetrate the bed of the stream in every
direction, and the water-cress, unable to obtain the requisite
amount of nourishment, gradually disappears.

THE STRUGGLE FOR EXISTENCE 25

Increase of Organisms in a Geometrical Ratio.

The facts which have now been adduced, sufficiently prove
that there is a continual competition, and struggle, and war
going on in nature, and that each species of animal and
plant affects many others in complex and often unexpected
ways. We will now proceed to show the fundamental cause
of this struggle, and to prove that it is ever acting over the
whole field of nature, and that no single species of animal or
plant can possibly escape from it. This results from the fact
of the rapid increase, in a geometrical ratio, of all the species
of animals and plants. In the lower orders this increase is
especially rapid, a single flesh-fly (Musca carnaria) producing
20,000 larvae, and these growing so quickly that they reach
their full size in five days ; hence the great Swedish naturalist,
Linnaeus, asserted that a dead horse would be devoured by three
of these flies as quickly as by a lion. Each of these larvae remains
in the pupa state about five or six days, so that each parent fly
may be increased ten thousand-fold in a fortnight. Supposing
they went on increasing at this rate during only three months
of summer, there would result one hundred millions of millions
of millions for each fly at the commencement of summer, — a
number greater probably than exists at any one time in the
whole world. And this is only one species, while there are
thousands of other species increasing also at an enormous rate ;
so that, if they were unchecked, the whole atmosphere would
be dense with flies, and all animal food and much of animal
life would be destroyed by them. To prevent this tremendous
increase there must be incessant war against these insects, by
insectivorous birds and reptiles as well as by other insects, in
the larva as well as in the perfect state, by the action of the
elements in the form of rain, hail, or drought, and by other
unknown causes ; yet we see nothing of this ever-present war,
though by its means alone, perhaps, we are saved from famine
and pestilence.

Let us now consider a less extreme and more familiar
case. We possess a considerable number of birds which,
like the redbreast, sparrow, the four common titmice, the
thrush, and the blackbird, stay with us all the year round.
These lay on an average six eggs, but, as several of them have

26 DARWINISM

two or more broods a year, ten will be below the average of
the year's increase. Such birds as these often live from fifteen
to twenty years in confinement, and we cannot suppose them to
live shorter lives in a state of nature, if unmolested ; but to
avoid possible exaggeration we will take only ten years as the
average duration of their lives. Now, if we start with a single
pair, and these are allowed to live and breed, unmolested, till
they die at the end of ten years, — as they might do if turned
loose into a good-sized island with ample vegetable and insect
food, but no other competing or destructive birds or quadrupeds
— their numbers would amount to more than twenty millions.
But we know very well that our bird population is no greater,
on the average, now than it was ten years ago. Year by year
it may fluctuate a little according as the winters are more
or less severe, or from other causes, but on the whole there is
no increase. What, then, becomes of the enormous surplus
population annually produced? It is evident they must
all die or be killed, somehow ; and as the increase is, on the
average, about five to one, it follows that, if the average
number of birds of all kinds in our islands is taken at ten
millions — and this is probably far under the mark — then about
fifty millions of birds, including eggs as possible birds, must
annually die or be destroyed. Yet we see nothing, or almost
nothing, of this tremendous slaughter of the innocents going
on all around us. In severe winters a few birds are found
dead, and a few feathers or mangled remains show us where
a wood-pigeon or some other bird has been destroyed by a
hawk, but no one would imagine that five times as many birds
as the total number in the country in early spring die every
year. No doubt a considerable proportion of these do not die
here but during or after migration to other countries, but others
which are bred in distant countries come here, and thus
balance the account. Again, as the average number of young
produced is four or five times that of the parents, we ought to
have at least five times as many birds in the country at the
end of summer as at the beginning, and there is certainly
no such enormous disproportion as this. The fact is, that the
destruction commences, and is probably most severe, with
nestling birds, which are often killed by heavy rains or blown
away by severe storms, or left to die of hunger if either of

ii THE STRUGGLE FOR EXISTENCE 27

the parents is killed ; while they offer a defenceless prey to
jackdaws, jays, and magpies, and not a few are ejected from
their nests by their foster-brothers the cuckoos. As soon as
they are fledged and begin to leave the nest great numbers
are destroyed by buzzards, sparrow-hawks, and shrikes. Of
those which migrate in autumn a considerable proportion are
probably lost at sea or otherwise destroyed before they reach a
place of safety ; while those which remain with us are greatly
thinned by cold and starvation during severe winters. Exactly
the same thing goes on with every species of wild animal and
plant from the lowest to the highest. All breed at such a rate,
that in a few years the progeny of any one species would, if
allowed to increase unchecked, alone monopolise the land ;
but all alike are kept within bounds by various destructive
agencies, so that, though the numbers of each may fluctuate,
they can never permanently increase except at the expense of
some others, which must proportionately decrease.

Cases showing the Great Powers of Increase of Animals.
As the facts now stated are the very foundation of the
theory we are considering, and the enormous increase and
perpetual destruction continually going on require to be kept
ever present in the mind, some direct evidence of actual cases
of increase must be adduced. That even the larger animals,
which breed comparatively slowly, increase enormously when
placed under favourable conditions in new countries, is shown
by the rapid spread of cattle and horses in America.
Columbus, in his second voyage, left a few black cattle at St.
Domingo, and these ran wild and increased so much that,
twenty-seven years afterwards, herds of from 4000 to 8000
head were not uncommon. Cattle were afterwards taken
from this island to Mexico and to other parts of America, and
in 1587, sixty -five years after the conquest of Mexico, the
Spaniards exported 64,350 hides from that country and
35,444 from St. Domingo, an indication of the vast numbers
of these animals which must then have existed there, since
those captured and killed could have been only a small portion
of the whole. In the pampas of Buenos Ayres there were, at
the end of the last century, about twelve million cows and
three million horses, besides great numbers in all other parts

28 DARWINISM

of America where open pastures offered suitable conditions.
Asses, about fifty years after their introduction, ran wild and
multiplied so amazingly in Quito, that the Spanish traveller
Ulloa describes them as being a nuisance. They grazed
together in great herds, defending themselves with their
mouths, and if a -horse strayed among them they all fell upon
him and did not cease biting and kicking till they left him
dead. Hogs were turned out in St. Domingo by Columbus
in 1493, and the Spaniards took them to other places where
they settled, the result being, that in about half a century
these animals were found in great numbers over a large part
of America, from 25° north to 40° south latitude. More
recently, in New Zealand, pigs have multiplied so greatly in
a wild state as to be a serious nuisance and injury to
agriculture. To give some idea of their numbers, it is stated
that in the province of Nelson there were killed in twenty
months 25,000 wild pigs.1 Now, in the case of all these animals,
we know that in their native countries, and even in America
at the present time, they do not increase at all in numbers ;
therefore the whole normal increase must be kept down,
year by year, by natural or artificial means of destruction.

Rapid Increase and Wide Spread of Plants.

In the case of plants, the power of increase is even greater
and its effects more distinctly visible. Hundreds of square
miles of the plains of La Plata are now covered Avith two or
three species of European thistle, often to the exclusion of
almost every other plant ; but in the native countries of these
thistles they occupy, except in cultivated or waste ground, a
very subordinate part in the vegetation. Some American
plants, like the cotton-weed (Asclepias curassavica), have now
become common weeds over a large portion of the tropics.
White clover (Trifolium repens) spreads over all the temperate
regions of the world, and in New Zealand is exterminating
many native species, including even the native flax (Phormium

1 Still more remarkable is the increase of rabbits both in New Zealand and
Australia. No less than seven millions of rabbit-skins have been exported
from the former country in a single year, their value being £67,000. In both
countries, sheep-runs have been greatly deteriorated in value by the abundance
of rabbits, which destroy the herbage ; and in some cases they have had to be
abandoned altogether.

ii THE STRUGGLE FOR EXISTENCE 29

tenax), a large plant with iris-like leaves 5 or 6 feet high.
Mr. W. L. Travers has paid much attention to the effects of
introduced plants in New Zealand, and notes the following
species as being especially remarkable. The common knot-
grass (Polygonum aviculare) grows most luxuriantly, single
plants covering a space 4 or 5 feet in diameter, and send-
ing their roots 3 or 4 feet deep. A large sub-aquatic
dock (Eumex obtusifolius) abounds in every river-bed, even
far up among the mountains. The common sow-thistle
(Sonchus oleraceus) grows all over the country up to an
elevation of 6000 feet. The water-cress (Nasturtium officinale)
grows with amazing vigour in many of the rivers, forming
stems 12 feet long and f inch in diameter, and completely
choking them up. It cost £300 a year to keep the Avon
at Christchurch free from it. The sorrel (Rumex acetosella)
covers hundreds of acres with a sheet of red. It forms a
dense mat; exterminating other plants, and preventing cultiva-
tion. It can, however, be itself exterminated by sowing the
ground with red clover, which will also vanquish the
Polygonum aviculare. The most noxious weed in New
Zealand appears, however, to be the Hypochseris radicata, a
coarse yellow -flowered composite not uncommon in our
meadows and waste places. This has been introduced with
grass seeds from England, and is very destructive. It is
stated that excellent pasture was in three years destroyed by
this weed, which absolutely displaced every other plant on the
ground. It grows in every kind- of soil, and is said even to
drive out the white clover, which is usually so powerful in
taking possession of the soil.

In Australia another composite plant, called there the Cape-
weed (Cryptostemma calendulaceum), did much damage, and was
noticed by Baron Von Hugel in 1833 as "an unexterminable
weed " ; but, after forty years' occupation, it was found to give
way to the dense herbage formed by lucerne and choice
grasses.

In Ceylon we are told by Mr. Thwaites, in his Enumera-
tion of Ceylon Plants, that a plant introduced into the
island less than fifty years ago is helping to alter the
character of the vegetation up to an elevation of 3000 feet.
This is the Lantana mixta, a verbenaceous plant introduced

30 DARWINISM

from the West Indies, which appears to have found in Ceylon a
soil and climate exactly suited to it. It now covers thousands
of acres with its dense masses of foliage, taking complete
possession of land where cultivation has been neglected or
abandoned, preventing the growth of any other plants, and
even destroying small trees, the tops of which its subscandent
stems are able to reach. The fruit of this plant is so accept-
able to frugivorous birds of all kinds that, through their instru-
mentality, it is spreading rapidly, to the complete exclusion of
the indigenous vegetation where it becomes established.

Great Fertility not essential to Rapid Increase.

The not uncommon circumstance of slow-breeding animals
being very numerous, shows that it is usually the amount
of destruction which an animal or plant is exposed to, not
its rapid multiplication, that determines its numbers in any
country. The passenger-pigeon (Ectopistes migratorius) is, or
rather was, excessively abundant in a certain area in North
America, and its enormous migrating flocks darkening the sky
for hours have often been described ; yet this bird lays only
two eggs. The fulmar petrel exists in myriads at St. Kilda
and other haunts of the species, yet it lays only one egg.
On the other hand the great shrike, the tree-creeper, the
nut-hatch, the nut-cracker, the hoopoe, and many other birds,
lay from four to six or seven eggs, and yet are never
abundant. So in plants, the abundance of a species bears
little or no relation to its seed-producing power. Some of the
grasses and sedges, the wild hyacinth, and many buttercups
occur in immense profusion over extensive areas, although each
plant produces comparatively few seeds ; while several species
of bell-flowers, gentians, pinks, and mulleins, and even some
of the composite, which produce an abundance of minute seeds,
many of which are easily scattered by the wind, are yet rare
species that never spread beyond a very limited area.

The above-mentioned passenger-pigeon affords such an
excellent example of an enormous bird-population kept up by
a comparatively slow rate of increase, and in spite of its
complete helplessness and the great destruction which it
suffers from its numerous enemies, that the following account
of one of its breeding-places and migrations by the celebrated

ir THE STRUGGLE FOR EXISTENCE 31

American naturalist, Alexander Wilson, will be read with
interest: —

" Not far from Shelby ville, in the State of Kentucky,
about five years ago, there was one of these breeding-places,
which stretched through the woods in nearly a north and
south direction, was several miles in breadth, and was said to
be upwards of 40 miles in extent. In this tract almost
every tree was furnished with nests wherever the branches
could accommodate them. The pigeons made their first
appearance there about the 10th of April, and left it
altogether with their young before the 25th of May. As
soon as the young were fully grown and before they left the
nests, numerous parties of the inhabitants from all parts of
the adjacent country came with waggons, axes, beds, cooking
utensils, many of them accompanied by the greater part of
their families, and encamped for several days at this immense
nursery. -Several of them informed me that the noise was
so great as to terrify their horses, and that it was difficult for
one person to hear another without bawling in his ear. The
ground was strewed with broken limbs of trees, eggs, and
young squab pigeons, which had been precipitated from above,
and on which herds of hogs were fattening. Hawks, buzzards,
and eagles were sailing about in great numbers, and seizing
the squabs from the nests at pleasure ; while, from 20 feet
upwards to the top of the trees, the view through the woods
presented a perpetual tumult of crowding and fluttering
multitudes of pigeons, their wings roaring like thunder,
mingled with the frequent crash of falling timber ; for now
the axemen were at work cutting down those trees that seemed
most crowded with nests, and contrived to fell them in such
a manner, that in their descent they might bring down several
others ; by which means the falling of one large tree some-
times produced 200 squabs little inferior in size to the old
birds, and almost one heap of fat. On some single trees
upwards of a hundred nests were found, each containing one
squab only ; a circumstance in the history of the bird not
generally known to naturalists.1 It was dangerous to walk

1 Later observers have proved that two eggs are laid and usually two
young produced, but it may be that in most cases only one of these comes to
maturity.

32 DARWINISM chap.

under these flying and fluttering millions, from the frequent
fall of large branches, broken down by the weight of the
multitudes above, and which in their descent often destroyed
numbers of the birds themselves ; while the clothes of those
engaged in traversing the woods were completely covered
with the excrements of the pigeons.

" These circumstances were related to me by many of the
most respectable part of the community in that quarter, and
were confirmed in part by what I myself witnessed. I passed
for several miles through this same breeding-place, where
every tree was spotted -with nests, the remains of those above
described. In many instances I counted upwards of ninety
nests on a single tree ; but the pigeons had abandoned this
place for another, 60 or 80 miles off, towards Green
River, where they were said at that time to be equally
numerous. From the great numbers that were constantly
passing over our heads to or from that quarter, I had no
doubt of the truth of this statement. The mast had been
chiefly consumed in Kentucky; and the pigeons, every morn-
ing a little before sunrise, set out for the Indiana territory,
the nearest part of which was about sixty miles distant.
Many of these returned before ten o'clock, and the great body
generally appeared on their return a little after noon. I had
left the public road to visit the remains of the breeding-place
near Shelbyville, and was traversing the woods with my gun,
on my way to Frankfort, when about ten o'clock the pigeons
which I had observed flying the greater part of the morning
northerly, began to return in such immense numbers as I never
before had witnessed. Coming to an opening by the side of
a creek, where I had a more uninterrupted view, I was
astonished at their appearance : they were flying with great
steadiness and rapidity, at a height beyond gunshot, in
several strata deep, and so close together that, could shot
have reached them, one discharge could not have failed to
bring down several individuals. From right to left, as far as
the eye could reach, the breadth of this vast procession ex-
tended, seeming everywhere equally crowded, Curious to
determine how long this appearance would continue, I took
out my watch to note the time, and sat down to observe them.
It was then half-past one ; I sat for more than an hour, but

it THE STRUGGLE FOR EXISTENCE 33

instead of a diminution of this prodigious procession, it seemed
rather to increase, both in numbers and rapidity ; and anxious
to reach Frankfort before night, I rose and went on. About
four o'clock in the afternoon I crossed Kentucky Eiver, at the
town of Frankfort, at which time the living torrent above my
head seemed as numerous and as extensive as ever. Long-
after this I observed them in large bodies that continued to
pass for six or eight minutes, and these again were followed
by other detached bodies, all moving in the same south-east
direction, till after six o'clock in the evening. The great
breadth of front which this mighty multitude preserved would
seem to intimate a corresponding breadth of their breeding-
place, which, by several gentlemen who had lately passed
through part of it, was stated to me at several miles."

From these various observations, Wilson calculated that
the number of birds contained in the mass of pigeons which
he saw on -this occasion was at least two thousand millions,
while this was only one of many similar aggregations known
to exist in various parts of the United States. The
picture here given of these defenceless birds, and their still
more defenceless young, exposed to the attacks of numerous
rapacious enemies, brings vividly before us one of the phases
of the unceasing struggle for existence ever going on ; but
when we consider the slow rate of increase of these birds,
and the enormous population they are nevertheless able to
maintain, we must be convinced that in the case of the
majority of birds which multiply far more rapidly, and yet
are never able to attain such numbers, the struggle against
their numerous enemies and against the adverse forces of
nature must be even more severe or more continuous.

Struggle for Life between closely allied Animals and Plants
often the most severe.

The struggle we have hitherto been considering has been
mainly that between an animal or plant and its direct enemies,
whether these enemies are other animals which devour it, or
the forces of nature which destroy it. But there is another
kind of struggle often going on at the same time between
closely related species, which almost always terminates in the
destruction of one of them. As an example of what is

D

34 DARWINISM

meant, Darwin states that the recent increase of the missel-
thrush in parts of Scotland has caused the decrease of the
song-thrush.1 The black rat (Mus rattus) was the common rat
of Europe till, in the beginning of the eighteenth century, the
large brown rat (Mus decumanus) appeared on the Lower
Volga, and thence spread more or less rapidly till it overran all
Europe, and generally drove out the black rat, which in most
parts is now comparatively rare or quite extinct. This invad-
ing rat has now been carried by commerce all over the world,
and in New Zealand has completely extirpated a native rat,
which the Maoris allege they brought with them from their
home in the Pacific ; and in the same country a native fly is
being supplanted by the European house-fly. In Russia the
small Asiatic cockroach has driven away a larger native species;
and in Australia the imported hive-bee is exterminating the
small stingless native bee.

The reason why this kind of struggle goes on is apparent
if we consider that the allied species fill nearly the same place
in the economy of nature. They require nearly the same
kind of food, are exposed to the same enemies and the same
dangers. Hence, if one has ever so slight an advantage over
the other in procuring food or in avoiding danger, in its
rapidity of multiplication or its tenacity of life, it will increase
more rapidly, and by that very fact will cause the -other to
decrease and often become altogether extinct. In some cases,
no doubt, there is actual war between the two, the stronger
killing the weaker; but this is by no means necessary, and
there may be cases in which the weaker species, physically,
may prevail, by its power of more rapid multiplication, its
better withstanding vicissitudes of climates, or its greater
cunning in escaping the attacks of the common enemies.
The same principle is seen at work in the fact that certain
mountain varieties of sheep will starve out other mountain
varieties, so that the two cannot be kept together. In plants
the same thing occurs. If several distinct varieties of wheat
are sown together, and the mixed seed resown, some of the
varieties which best suit the soil and climate, or are naturally
the most fertile, will beat the others and so yield more seed, and
will consequently in a few years supplant the other varieties.

1 Origin of Sj/ecies, p. 59. Professor A. Newton, however, informs me that
these species do not interfere with one another in the way here stated.

ir THE STRUGGLE FOR EXISTENCE 35

As an effect of this principle, we seldom find closely allied
species of animals or plants living together, but often in
distinct though adjacent districts where the conditions of life
are somewhat different. Thus we may find cowslips (Primula
veris) growing in a meadow, and primroses (P. vulgaris) in an
adjoining wood, each in abundance, but not often intermingled.
And for the same reason the old turf of a pasture or heath
consists of a great variety of plants matted together, so much
so that in a patch little more than a yard square Mr. Darwin
found twenty distinct species, belonging to eighteen distinct
genera and to eight natural orders, thus showing their extreme
diversity of organisation. For the same reason a number of
distinct grasses and clovers are sown in order to make a good
lawn instead of any one species ; and the quantity of hay
produced has been found to be greater from a variety of very
distinct grasses than from any one species of grass.

It may be thought that forests are an exception to this
rule, since in the north-temperate and arctic regions we find
extensive forests of pines or of oaks. But these are, after all,
exceptional, and characterise those regions only where the
climate is little favourable to forest vegetation. In the
tropical and all the warm temperate parts of the earth, where
there is a sufficient supply of moisture, the forests present the
same variety of species as does the turf of our old pastures ;
and in the equatorial virgin forests there is so great a variety
of forms, and they are so thoroughly intermingled, that the
traveller often finds it difficult to discover a second specimen
of any particular species which he has noticed. Even the
forests of the temperate zones, in all favourable situations,
exhibit a considerable variety of trees of distinct genera and
families, and it is only when we approach the outskirts of
forest vegetation, where either drought or winds or the severity
of the winter is adverse to the existence of most trees, that
we find extensive tracts monopolised by one or two species.
Even Canada has more than sixty different forest trees, and
the Eastern United States a hundred and fifty ; Europe is
rather poor, containing about eighty trees only ; while the
forests of Eastern Asia, Japan, and Manchuria are exceedingly
rich, about a hundred and seventy species being already
known. And in all these countries the trees grow inter-

36 DARWINISM

mingled, so that in every extensive forest we have a consider-
able variety, as may be seen in the few remnants of our
primitive Avoods in some parts of Epping Forest and the
New Forest.

Among animals the same law prevails, though, owing to
their constant movements and power of concealment, it is not
so readily observed. As illustrations Ave may refer to the
wolf, ranging over Europe and Northern Asia, while the jackal
inhabits Southern Asia and Northern Africa ; the tree-
porcupines, of which there are two closely allied species, one
inhabiting the eastern, the other the western half of North
America ; the common hare (Lepus timidus) in Central and
Southern Europe, while all Northern Europe is inhabited by
the variable hare (Lepus variabilis) ; the common jay (Garrulus
glandarius) inhabiting all Europe, while another species
(Garrulus Brandti) is found all across Asia from the Urals to
Japan ; and many species of birds in the Eastern United
States are replaced by closely allied species in the west. Of
course there are also numbers of closely related species in the
same country, but it will almost always be found that they
frequent different stations and have somewhat different habits,
and so do not come into direct competition with each other ;
just as closely allied plants may inhabit the same districts,
when one prefers meadows the other woods, one a chalky
soil the other sand, one a damp situation the other a dry one.
With plants, fixed as they are to the earth, Ave easily note
these peculiarities of station ; but Avith Avild animals, Avhich we
see only on rare occasions, it requires close and long-continued
observation to detect the peculiarities in their mode of life
Avhich may prevent all direct competition betAveen closely
allied species dAvelling in the same area.

The Ethical Aspect of the Struggle for Existence.

Our exposition of the phenomena presented by the struggle
for existence may be fitly concluded by a feAV remarks on its
ethical aspect. Now that the war of nature is better knoAvn,
it has been dAvelt upon by many AAriters as presenting so vast
an amount of cruelty and pain as to be revolting to our
instincts of humanity, Avhile it has proved a stumbling-block
in the Avay of those Avho Avould fain believe in an all-Avise and

ii THE STRUGGLE FOR EXISTENCE 37

benevolent ruler of the universe. Thus, a brilliant writer
says : " Pain, grief, disease, and death, are these the inventions
of a lovina; God? That no animal shall rise to excellence
except by being fatal to the life of others, is this the law of
a kind Creator? It is useless to say that pain has its
benevolence, that massacre has its mercy. Why is it so
ordained that bad should be the raAV material of good ? Pain
is not the less pain because it is useful ; murder is not less
murder because it is conducive to development. Here is
blood upon the hand still, and all the perfumes of Arabia will
not sweeten it."1

Even so thoughtful a writer as Professor Huxley adopts
similar views. In a recent article on " The Struggle for
Existence " he speaks of the myriads of generations of herbiv-
orous animals which " have been tormented and devoured by
carnivores " ; of the carnivores and herbivores alike " subject to
all the miseries incidental to old age, disease, and over-multi-
plication"; and of the "more or less enduring suffering,"
which is the meed of both vanquished and victor. And he
concludes that, since thousands of times a minute, were our
ears sharp enough, we should hear sighs and groans of pain
like those heard by Dante at the gate of hell, the world
cannot be governed by what we call benevolence.2

Now there is, I think, good reason to believe that all this
is greatly exaggerated ; that the supposed " torments " and
" miseries " of animals have little real existence, but are the
reflection of the imagined sensations of cultivated men and
Avomen in similar circumstances ; and that the amount of actual
suffering caused by the struggle for existence among animals
is altogether insignificant. Let us, therefore, endeavour to
ascertain what are the real facts on which these tremendous
accusations are founded.

In the first place, we must remember that animals are
entirely spared the pain we suffer in the anticipation of death —
a pain far greater, in most cases, than the reality. This leads,
probably, to an almost perpetual enjoyment of their lives ;
since their constant watchfulness against danger, and even
their actual flight from an enemy, will be the enjoyable

1 Winwood Reade's Martyrdom of Man, p. 520.

2 Nineteenth Century, February 1888, pp. 162, 163.

DARWINISM

exercise of the powers and faculties they possess, unmixed
with any serious dread. There is, in the next place, much
evidence to show that violent deaths, if not too prolonged, are
painless and easy ; even in the case of man, whose nervous
system is in all probability much more susceptible to pain than
that of most animals. In all cases in which persons have
escaped after being seized by a lion or tiger, they declare
that they suffered little or no pain, physical or mental. A
well-known instance is that of Livingstone, who thus describes
his sensations when seized by a lion : " Starting and looking
half round, I saw the lion just in the act of springing on me.
I was upon a little height ; he caught my shoulder as he sprang,
and we both came to the ground below together. Growling
horribly close to my ear, he shook me as a terrier-dog does a
rat. The shock produced a stupor similar to that which
seems to be felt by a mouse after the first shake of the cat.
It causes a sort of dreaminess, in which there was no sense of
pain or feeling of terror, though I was quite conscious of all
that was happening. It was like what patients partially
under the influence of chloroform describe, who see all the
operation, but feel not the knife. This singular condition
was not the result of any mental process. The shake
annihilated fear, and allowed no sense of horror in looking
round at the beast."

This absence of pain is not peculiar to those seized by wild
beasts, but is equally produced by any accident which causes
a general shock to the system. Mr. Whymper describes an
accident to himself during one of his preliminary explorations
of the Matterhorn, when he fell several hundred feet, bounding
from rock to rock, till fortunately embedded in a snow-drift
near the edge of a tremendous precipice. He declares that
while falling and feeling bloAV after Uoav, he neither lost
consciousness nor suffered pain, merely thinking, calmly, that
a few more blows would finish him. We have therefore a
right to conclude, that when death follows soon after any
great shock it is as easy and painless a death as possible ; and
this is certainly what happens when an animal is seized by a
beast of prey. For the enemy is one which hunts for food,
not for pleasure or excitement ; and it is doubtful whether any
carnivorous animal in a state of nature begins to seek after

ii THE STRUGGLE FOR EXISTENCE 39

prey till driven to do so by hunger. When an animal is
caught, therefore, it is very soon devoured, and thus the first
shock is followed by an almost painless death. Neither do
those which die of cold or hunger suffer much. Cold is
generally severest at night and has a tendency to produce
sleep and painless extinction. Hunger, on the other hand, is
hardly felt during periods of excitement, and when food is
scarce the excitement of seeking for it is at its greatest. It
is probable, also, that when hunger presses, most animals will
devour anything to stay their hunger, and will die of gradual
exhaustion and weakness not necessarily painful, if they do
not fall an earlier prey to some enemy or to cold.1

Now let us consider what are the enjoyments of the lives
of most animals. As a rule they come into existence at a
time of year when food is most plentiful and the climate most
suitable, that is in the spring of the temperate zone and at
the commencement of the dry season in the tropics. They
grow vigorously, being supplied with abundance of food ; and
when they reach maturity their lives are a continual round of
healthy excitement and exercise, alternating with complete
repose. The daily search for the daily food employs all their
faculties and exercises every organ of their bodies, while this
exercise leads to the satisfaction of all their physical needs.
In our own case, we can give no more perfect definition of
happiness, than this exercise and this satisfaction ; and we
must therefore conclude that animals, as a rule, enjoy all the
happiness of which they are capable. And this normal state
of happiness is not alloyed, as with us, by long periods —
whole lives often — of poverty or ill-health, and of the un-
satisfied longing for pleasures which others enjoy but to which
we cannot attain. Illness, and what answers to poverty in
animals — continued hunger — are quickly followed by unantici-
pated and almost painless extinction. Where we err is, in
giving to animals feelings and emotions which they do not
possess. To us the very sight of blood and of torn or mangled
limbs is painful, while the idea of the suffering implied by it

1 The Kestrel, which usually feeds on mice, birds, and frogs, sometimes
stays its hunger with earthworms, as' do some of the American buzzards.
The Honey-buzzard sometimes eats not only earthworms and slugs, but even
corn ; and the Buteo borealis of North America, whose usual food is small
mammals and birds, sometimes eats crayfish.

40 DARWINISM

is heartrending. We have a horror of all violent and sudden
death, because we think of the life full of promise cut short,
of hopes and expectations unfulfilled, and of the grief of
mourning relatives. But all this is quite out of place in the
case of animals, for whom a violent and a sudden death is in
every way the best. Thus the poet's picture of

"Nature red in tooth and claw
With ravine "

is a picture the evil of which is read into it by our
imaginations, the reality being made up of full and happy
lives, usually terminated by the quickest and least painful of
deaths.

On the whole, then, we conclude that the popular idea of
the struggle for existence entailing misery and pain on the
animal Avorld is the very reverse of the truth. What it
really brings about, is, the maximum of life and of the enjoy-
ment of life with the minimum of suffering and pain. Given
the necessity of death and reproduction — and without these
there could have been no progressive development of the
organic Avorld, — and it is difficult even to imagine a system
by which a greater balance of happiness could have been
secured. And this view was evidently that of Darwin himself,
who thus concludes his chapter on the struggle for existence :
" When we reflect on this struggle, we may console ourselves
with the full belief that the war of nature is not incessant,
that no fear is felt, that death is generally prompt, and that
the vigorous, the healthy, and the happy survive and
multiply."

CHAPTER III

THE VARIABILITY OF SPECIES IN A STATE OF NATURE

Importance of variability — Popular ideas regarding it — Variability of the
lower animals — The variability of insects — Variation among lizards
— Variation among birds — Diagrams of bird-variation — Number of
varying individuals — Variation in the mammalia— Variation in
internal organs — Variations in the skull — Variations in the habits
of Animals — The Variability of plants — Species which vary little —
Concluding remarks.

The foundation of the Darwinian theory is the variability of
species, and it is quite useless to attempt even to understand
that theory, much less to appreciate the completeness of the
proof of it, unless we first obtain a clear conception of the
nature and extent of this variability. The most frequent and
the most misleading of the objections to the efficacy of natural
selection arise from ignorance of this subject, an ignorance
shared by many naturalists, for it is only since Mr. Darwin
has taught us their importance that varieties have been
systematically collected and recorded ; and even now very
few collectors or students bestow upon them the attention
they deserve. By the older naturalists, indeed, varieties —
especially if numerous, small, and of frequent occurrence —
were looked upon as an unmitigated nuisance, because they
rendered it almost impossible to give precise definitions of
species, then considered the chief end of systematic natural
history. Hence it was the custom to describe what was
supposed to be the " typical form " of species, and most
collectors were satisfied if they possessed this typical form
in their cabinets. Now, however, a collection is valued in
proportion as it contains illustrative specimens of all the
varieties that occur in each species, and in some cases these

42 DARWINISM

have been carefully described, so that Ave possess a consider-
able mass of information on the subject. Utilising this in-
formation we will now endeavour to give some idea of the
nature and extent of variation in the species of animals and
plants.

It is very commonly objected that the widespread and
constant variability which is admitted to be a characteristic of
domesticated animals and cultivated plants is largely due to
the unnatural conditions of their existence, and that we have
no proof of any corresponding amount of variation occurring
in a state of nature. Wild animals and plants, it is said, are
usually stable, and when variations occur these are alleged to
be small in amount and to affect superficial characters only ;
or if larger and more important, to occur so rarely as not to
afford any aid in the supposed formation of new species.

This objection, as will be shown, is utterly unfounded ;
but as it is one which goes to the very root of the problem, it
is necessary to enter at some length into the various proofs of
variation in a state of nature. This is the more necessary
because the materials collected by Mr. Darwin bearing on
this question have never been published, and comparatively
few of them have been cited in The Origin of Species ; while a
considerable body of facts has been made known since the
publication of the last edition of that work.

Variability of the Lower Animals.

Among the lowest and most ancient marine organisms are
the Foraminifera, little masses of living jelly, apparently
structureless, but which secrete beautiful shelly coverings,
often perfectly symmetrical, as varied in form as those of the
mollusca and far more complicated. These have been studied
with great care by many eminent naturalists, and the late Dr.
W. B. Carpenter in his great work — the Introduction to the
Study of the Foraminifera— thus refers to their variability :
"There is not a single species of plant or animal of which the
range of variation has been studied by the collocation and
comparison of so large a number of specimens as have passed
under the review of Messrs. Williamson, Parker, Rupert
Jones, and myself in our studies of the types of this group;"
and he states as the result of this extensive comparison of

in VARIABILITY OF SPECIES IN A STATE OF NATURE 43

specimens : " The range of variation is so great among the
Foraminifera as to include not merely those differential char-
acters which have been usually accounted specific, but also
those upon which the greater part of the genera of this group
have been founded, and even in some instances those of its
orders." 1

Coming now to a higher group — the Sea- Anemones — Mr. P.
H. Gosse and other writers on these creatures often refer to
variations in size, in the thickness and length of the tentacles,
the form of the disc and of the mouth, and the character of
surface of the column, while the colour varies enormously in
a great number of the species. Similar variations occur in all
the various groups of marine invertebrata, and in the great
sub-kingdom of the mollusca they are especially numerous.
Thus, Dr. S. P. Woodward states that many present a most
perplexing amount of variation, resulting (as he supposes)
from supply of food, variety of depth and of saltness of the
water ; but we know that many variations are quite inde-
pendent of such causes, and we will now consider a few cases
among the land-mollusca in which they have been more care-
fully studied.

In the small forest region of Oahu, one of the Sandwich
Islands, there have been found about 175 species of land-shells
represented by 700 or 800 varieties ; and we are told by the
Rev. J. T. Gulick, who studied them carefully, that "we
frequently find a genus represented in several successive
valleys by allied species, sometimes feeding on the same, some-
times on different plants. In every such case the valleys
that are nearest to each other furnish the most nearly allied
forms ; and a full set of the varieties of each species presents
a minute gradation of forms between the more divergent types
found in the more widely separated localities."

In most land-shells there is a considerable amount of varia-
tion in colour, markings; size, form, and texture or striation
of the surface, even in specimens collected in the same
locality. Thus, a French author has enumerated no less than
198 varieties of the common wood -snail (Helix nemoralis),
while of the equally common garden -snail (Helix hortensis)
ninety varieties have been described. Fresh-water shells are also
1 Foraminifera, preface, p. x.

44 DARWINISM

subject to great variation, so that there is much uncertainty as
to the number of species ; and variations are especially frequent
in the Planorbidse, which exhibit many eccentric deviations from
the usual form of the species — deviations which must often
affect the form of the living animal. In Mr. Ingersoll's Eeport
on the Recent Mollusca of Colorado many of these extra-
ordinary variations are referred to, and it is stated that a shell
(Helisonia trivolvis) abundant in some small ponds and lakes,
had scarcely two specimens alike, and many of them closely
resembled other and altogether distinct species.1

The Variability of Insects.

Among Insects there is a large amount of variation, though
very few entomologists devote themselves to its investigation.
Our first examples will be taken from the late Mr. T. Vernon
Wollaston's book, On the Variation of Species, and they
must be considered as indications of very widespread though
little noticed phenomena. He speaks of the curious little
carabideous beetles of the genus Notiophilus as being
" extremely unstable both in their sculpture and hue ; " of
the common Calathus mollis as having "the hind wings at
one time ample, at another rudimentary, and at a third nearly
obsolete ; " and of the same irregularity as to the wings being
characteristic of many Orthoptera and of the Homopterous
Fulgoridse. Mr. Westwood in his Modern Classification of
Insects states that. "the species of Gerris, Hydrometra, and
Velia are mostly found perfectly apterous, though occasionally
with full-sized wings."

It is, however, among the Lepidoptera (butterflies and
moths) that the most numerous cases of variation have been
observed, and every good collection of these insects affords
striking examples. I will first adduce the testimony of Mr.
Bates, who speaks of the butterflies of the Amazon valley
exhibiting innumerable local varieties or races, while some
species showed great individual variability. Of the beautiful
Mechanitis Polymnia he says, that at Ega on the Upper
Amazons, " it varies not only in general colour and pattern,
but also very considerably in the shape of the wings,
especially in the male sex." Again, at St. Paulo, Ithomia
1 United States Geological Survey of the Territories, 1874.

m VARIABILITY OF SPECIES IN A STATE OF NATURE 45

Orolina exhibits four distinct varieties, all occurring together,
and these differ not only in colour but in form, one variety
being described as having the fore wings much elongated in the
male, while another is much larger and has "the hind wings in
the male different in shape." Of Heliconius Numata Mr. Bates
says: "This species is so variable that it is difficult to find two
examples exactly alike," while " it varies in structure as well
as in colours. The wings are sometimes broader, some-
times narrower ; and their edges are simple in some examples
and festooned in others." Of another species of the same
genus, H. melpomene, ten distinct varieties are described
all more or less connected by intermediate forms, and four
of these varieties were obtained at one locality, Serpa on
the north bank of the Amazon. Ceratina Ninonia is another
of these very unstable species exhibiting many local varieties
which are, however, incomplete and connected by intermediate
forms ; while the several species of the genus Lycorea all
vary to such an extent as almost to link them together, so
that Mr. Bates thinks they might all fairly be considered as
varieties of one species only.

Turning to the Eastern Hemisphere we have in Papilio
Severus a species which exhibits a large amount of simple
variation, in the presence or absence of a pale patch on the
upper wings, in the brown submarginal marks on the lower
wings, in the form and extent of the yellow band, and in
the size of the specimens. The most extreme forms, as well
as the intermediate ones, are often found in one locality and
in company with each other. A small butterfly (Terias hecabe)
ranges over the whole of the Indian and Malayan regions to
Australia, and everywhere exhibits great variations, many of
which have been described as distinct species ; but a gentle-
man in Australia bred two of these distinct forms (T. hecabe
and T. iEsiope), with several intermediates, from one batch of
caterpillars found feeding together on the same plant.1 It is
therefore very probable that a considerable number of supposed
distinct species are only individual varieties.

Cases of variation similar to those now adduced among
butterflies might be increased indefinitely, but it is as well to
note that such important characters as the neuration of the
1 Proceedings of the Entomological Society of London, 1875, p. vii.

46 DARWINISM

wings, on which generic and family distinctions are often
established, are also subject to variation. The Eev. K. P.
Murray, in 1872, laid before the Entomological Society
examples of such variation in six species of butterflies, and
other cases have been since described. The larvae of butter-
flies and moths are also very variable, and one observer
recorded in the Proceedings of the Entomological Society for
1870 no less than sixteen varieties of the caterpillar of the
bedstraw hawk-moth (Deilephela galii).

Variation among Lizards.

Passing on from the lower animals to the vertebrata, we
find more abundant and more definite evidence as to the
extent and amount of individual variation. I will first give a
case among the Keptilia from some of Mr. Darwin's un-
published MSS., which have been kindly lent me by Mr.
Francis Darwin.

"M. Milne Edwards (Annates des Sci. Nat., 1 ser., torn,
xvi. p. 50) has given a curious table of measurements of four-
teen specimens of Lacerta muralis ; and, taking the length of
the head as a standard, he finds the neck, trunk, tail, front
and hind legs, colour, and femoral pores, all varying wonder-
fully ; and so it is more or less with other species. So ap-
parently trifling a character as the scales on the head affording
almost the only constant characters."

As the table of measurements above referred to would give
no clear conception of the nature and amount of the variation
without a laborious study and comparison of the figures, I
have endeavoured to find a method of presenting the facts to
the eye, so that they may be easily grasped and appreciated.
In the diagram opposite, the comparative variations of the
different organs of this species are given by means of variously
bent lines. The head is represented by a straight line because
it presented (apparently) no variation. The body is next
given, the specimens being arranged in the order of their size
from No. 1, the smallest, to No. 14, the largest, the actual
lengths being laid down from a base line at a suitable
distance below, in this case two inches below the centre, the
mean length of the body of the fourteen specimens being two
inches. The respective lengths of the neck, legs, and toe of

DIAGRAM OF VARIATION

47

7 3 5 7 9 n n

Head-

Body

Mean length. 2in.

Neck-

Mean length. 1. 18in.

Fore Leg.

Mean length. 1.05in.

Hind Leg.

Mean length. 1. 90in,

Toe of Hind Foot.

Mean length. 0. 70in.

y

\

T^ 3 5 7 9 11 14

The lengths in the table are given in millimetres, which are here reduced
to inches for the means.

Fia. 1.— Variations of Laccrta muralis.

DARWINISM

W _ Neck

Lacerta ocellata

lan^ - -..Hind Legs

[in H miiiiiiiiiiiiiiim— ■ imiimii i | Tail

ft Neck

Lacerta uiridis ' "" ° y. ,

#.._ Hind Legs

ft— — — a ft 7W//

ft*.— JVec/f

*mm.:.....Body

Lacerta agilis

M Hind Legs

!«■( ii/ecft

Lacerta mural is

>"■ ■■ ■■■' ■ ii iiiiini ;r 7a/7

I..- /Vecft

Lacerta uelox , . " „. , ,

^snt... Hind Legs

m. Tail

M - - Heck

Lacerta deserti " ,,. , ,

[M Hind Legs

!■ »" ■ — ■—■—■— 1 7a //

Length of Head friiiiiiiiiiimii«iiiiiMiiiiiiii| taAe/? as the
standard in each of the above -named species

Fia. 2. — Variation of Lizards,

nr VARIABILITY OF SPECIES IN A STATE OF NATURE 49

each specimen are then laid down in the same manner at
convenient distances apart for comparison ; and we see that
their variations bear no definite relation to those of the body,
and not much to those of each other. With the exception of
No. 5, in which all the parts agree in being large, there is a
marked independence of each part, shown by the lines often
curving in opposite directions ; which proves that in those
specimens one part is large while the other is small. The
actual amount of the variation is very great, ranging from
one-sixth of the mean length in the neck to considerably more
than a fourth in the hind leg, and this among only fourteen
examples which happen to be in a particular museum.

To prove that this is not an isolated case, Professor Milne
Edwards also gives a table showing the amount of variation in
the museum specimens of six common species of lizards, also
taking the- head as the standard, so that the comparative
variation of each part to the head is given. In the accompany-
ing diagram (Fig. 2) the variations are exhibited by means of
lines of varying length. It will be understood that, however
much the specimens varied in size, if they had kept the same
proportions, the variation line would have been in every case
reduced to a point, as in the neck of L. velox which exhibits
no variation. The different proportions of the variation lines
for each species may show a distinct mode of variation, or may
be merely due to the small and differing number of specimens ;
for it is certain that whatever amount of variation occurs
among a few specimens will be greatly increased when a much
larger number of specimens are examined. That the amount of
variation is large, may be seen by comparing it with the actual
length of the head (given below the diagram) which was used
as a standard in determining the variation, but which itself
seems not to have varied.1

Variation among Birds.

Coming now to the class of Birds, we find much more
copious evidence of variation. This is due partly to the fact
that Ornithology has perhaps a larger body of devotees than
any other branch of natural history (except entomology) ; to
the moderate size of the majority of birds ; and to the circum-
1 Arm. des Sci. Nat., torn. xvi. p. 50.
E

50 DARWINISM

stance that the form and dimensions of the wings, tail, beak,
and feet offer the best generic and specific characters and can
all be easily measured and compared. The most systematic
observations on the individual variation of birds have been
made by Mr. J. A. Allen, in his remarkable memoir : " On the
Mammals and Winter Birds of East Florida, with an examina-
tion of certain assumed specific characters in Birds, and a
sketch of the Bird Faunae of Eastern North America,"
published in the Bulletin of the Museum of Comparative
Zoology at Harvard College, Cambridge, Massachusetts, in
1871. In this work exact measurements are given of all the
chief external parts of a large number of species of common
American birds, from twenty to sixty or more specimens of
each species being measured, so that we are able to determine
with some precision the nature and extent of the variation
that usually occurs. Mr. Allen says : " The facts of the
case show that a variation of from 15 to 20 per cent
in general size, and an equal degree of variation in the
relative size of different parts, may be ordinarily expected
among specimens of the same species and sex, taken at the
same locality, while in some cases the variation is even greater
than this." He then goes on to show that each part varies
to a considerable extent independently of the other parts ; so
that when the size varies, the proportions of all the parts
vary, often to a much greater amount. The wing and tail,
for example, besides varying in length, vary in the pro-
portionate length of each feather, and this causes their outline
to vary considerably in shape. The bill also varies in length,
width, depth, and curvature. The tarsus varies in length, as
does each toe separately and independently ; and all this not
to a minute degree requiring very careful measurement to
detect it at all, but to an amount easily seen without any
measurement, as it averages one-sixth of the whole length and
often reaches one -fourth. In twelve species of common
perching birds the wing varied (in from twenty-five to thirty
specimens) from 14 to 21 per cent of the mean length, and the
tail from 13-8 to 2 3 "4 per cent. The variation of the form of
the wing can be very easily tested by noting which feather is
longest, which next in length, and so on, the respective
feathers being indicated by the numbers 1, 2, 3, etc., com-

in VARIABILITY OF SPECIES IN A STATE OF NATURE 51

mencing with the outer one. As an example of the irregular
variation constantly met with, the following occurred among
twenty -five specimens of Dendroeca coronata. Numbers
bracketed imply that the corresponding feathers were of
equal length.1

Relative Lengths of Primary Wing Feathers of
Dendrceca coronata.

Longest.

Second in
Length.

Third in
Length.

Fourth in
Length.

Fifth in
Length.

Sixth in
Length.

2

3

1

4

5

6

3

2

4

1

5

6

3

I I

1

5

6

7

* f

4

1

5

6

7

? )

■I

5

6

7

8

9

Here we have five very distinct proportionate lengths of
the wing feathers, any one of which is often thought sufficient
to characterise a distinct species of bird ; and though this is
rather an extreme case, Mr. Allen assures us that " the com-
parison, extended in the table to only a few species, has been
carried to scores of others with similar results."

Along with this variation in size and proportions there occurs
a large amount of variation in colour and markings. "The
difference in intensity of colour between the extremes of a
series of fifty or one hundred specimens of any species, collected
at a single locality, and nearly at the same season of the year,
is often as great as occurs between truly distinct species." But
there is also a great amount of individual variability in the
markings of the same species. Birds having the plumage
varied with streaks and spots differ exceedingly in different
individuals of the same species in respect to the size, shape,
and number of these marks, and in the general aspect of the
plumage resulting from such variations. " In the common

1 See Winter Birds of Florida, p. 206, Table F.

52 DARWINISM

song sparrow (Melospiza melodia), the fox-coloured sparrow
(Passerella iliaca), the swamp sparrow (Melospiza palustris), the
black and white creeper (Mniotilta varia), the water-wagtail
(Seiurus noveeboracencis), in Turdus fuscescens and its allies, the
difference in the size of the streaks is often very considerable.
In the song sparrow they vary to such an extent that in some
cases they are reduced to narrow lines ; in others so enlarged
as to cover the greater part of the breast and sides of the body,
sometimes uniting on the middle of the breast into a nearly
continuous patch."

Mr. Allen then goes on to particularise several species in
which such variations occur, giving cases in which two speci-
mens taken at the same place on the same day exhibited the
two extremes of coloration. Another set of variations is
thus described : " The white markings so common on the wings
and tails of birds, as the bars formed by the white tips of the
greater wing-coverts, the white patch occasionally present at
the base of the primary quills, or the white band crossing
them, and the white patch near the end of the outer tail-
feathers are also extremely liable to variation in respect to
their extent and the number of feathers to which, in the same
species, these markings extend." It is to be especially noted
that all these varieties are distinct from those which depend
on season, on age, or on sex, and that they are such as have
in many other species been considered to be of specific
value.

These variations of colour could not be presented to the eye
without a series of carefully engraved plates, but in order to
bring Mr. Allen's measurements, illustrating variations of size and
proportion, more clearly before the reader, I have prepared a
series of diagrams illustrating the more important facts and
their bearings on the Darwinian theory.

The first of these is intended, mainly, to show the actual
amount of the variation, as it gives the true length of the
wing and tail in the extreme cases among thirty specimens of
each of three species. The shaded portion shows the minimum
length, the unshaded portion the additional length in the
maximum. The point to be specially noted here is, that in
each of these common species there is about the same amount
of variation, and that it is so great as to be obvious at a glance.

Ill

DIAGRAM OF VARIATION

53

&

-to

n <*>

^

"53

^ t.

54 DARWINISM

There is here no question of " minute " or " infinitesimal "
variation, which many people suppose to be the only kind of
variation that exists. It cannot even be called small; yet
from all the evidence we now possess it seems to be the
amount which characterises most of the common species of
birds.

It maybe said, however, that these are the extreme variations,
and only occur in one or two individuals, while the great
majority exhibit little or no difference. Other diagrams will
show that this is not the case ; but even if it were so, it would
be no objection at all, because these are the extremes among
thirty specimens only. We may safely assume that these thirty
specimens, taken by chance, are not, in the case of all these
species, exceptional lots, and therefore we might expect at least
two similarly varying specimens in each additional thirty. But
the number of individuals, even in a very rare species, is
probably thirty thousand or more, and in a common species
thirty, or even three hundred, millions. Even one individual
in each thirty, varying to the amount shown in the diagram,
would give at least a million in the total population of any
common bird, and among this million many would vary much
more than the extreme among thirty only. We should thus
have a vast body of individuals varying to a large extent in
the length of the wings and tail, and offering ample material
for the modification of these organs by natural selection. We
will now proceed to show that other parts of the body vary,
simultaneously, but independently, to an equal amount.

The first bird taken is the common Bob-o-link or Rice-bird
(Dolichonyx oryzivorus), and the Diagram, Fig. 4, exhibits the
variations of seven important characters in twenty male adult
specimens.1 These characters are — the lengths of the body,
wing, tail, tarsus, middle toe, outer toe, and hind toe, being as
many as can be conveniently exhibited in one diagram. The
length of the body is not given by Mr. Allen, but as it forms
a convenient standard of comparison, it has been obtained by
deducting the length of the tail from the total length of the
birds as given by him. The diagram has been constructed
as follows : — The twenty specimens are first arranged in a
series according to the body-lengths (which may be con-
1 See Table I, p. 211, of Allen's Winter Birds of Florida.

DIAGRAM OF VARIATION

55

15 10 15 20

Fio. 4.— Dolichonyx oryzivorus. 20 Males.

56

DARWINISM

CHAP.

/ 5 70 15 20 25 30 35 40

1 5

10 15 20 25 30

Fio. 5.— Agelrens phceniceus. 40 Males.

40

in VARIABILITY OF SPECIES IN A STATE OF NATURE 57

sidered to give the size of the bird), from the shortest
to the longest, and the same number of vertical lines are
drawn, numbered from one to twenty. In this case (and
wherever practicable) the body-length is measured from the
lower line of the diagram, so that the actual length of the bird
is exhibited as well as the actual variations of length. These
can be well estimated by means of the horizontal line drawn
at the mean between the two extremes, and it will be seen
that one-fifth of the total number of specimens taken on either
side exhibits a very large amount of variation, which would of
course be very much greater if a hundred or more specimens
were compared. The lengths of the wing, tail, and other parts
are then laid down, and the diagram thus exhibits at a glance
the comparative variation of these parts in every specimen as
well as the actual amount of variation in the twenty specimens ;
and we are thus enabled to arrive at some important con-
clusions.

We note, first, that the variations of none of the parts follow
the variations of the body, but are sometimes almost in an
opposite direction. Thus the longest wing corresponds to a
rather small body, the longest tail to a medium body, while
the longest leg and toes belong to only a moderately large body.
Again, even related parts do not constantly vary together but
present many instances of independent variation, as shown by
the want of parallelism in their respective variation-lines. In
No. 5 (see Fig. 4) the wing is very long, the tail moderately
so ; while in No. 6 the wing is much shorter while the tail is
considerably longer. The tarsus presents comparatively little
variation ; and although the three toes may be said to vary in
general together, there are many divergencies ; thus, in passing
from No. 9 to No. 10, the outer toe becomes longer, while the
hind toe becomes considerably shorter ; while in Nos. 3 and 4
the middle toe varies in an opposite way to the outer and the
hind toes.

In the next diagram (Fig. 5) we have the variations in
forty males of the Red-winged Blackbird (Agelaeus phceniceus),
and here we see the same general features. One-fifth of the
whole number of specimens offer a large amount of variation
either below or above the mean ; while the wings, tail, and head
vary quite independently of the body. The wing and tail too,

58

DARWINISM

10 15

25 30

15 10 15 20 25 30

Fia, 6. — Cardinally virginianus. 31 Males.

in VARIABILITY OF SPECIES IN A STATE OF NATURE 59

though showing some amount of correlated variation, yet in
no less than nine cases vary in opposite directions as compared
with the preceding species.

The next diagram (Fig. 6), showing the variations of thirty-
one males of the Cardinal bird (Cardinalis virginianus), exhibits
these features much more strongly. The amount of variation
in proportion to the size of the bird is very much greater ;
while the variations of the wing and tail not only have no
correspondence with that of the body but very little with each
other. In no less than twelve or thirteen instances they vary
in opposite directions, while even where they correspond in
direction the amount of the variation is often very dispropor-
tionate.

As the proportions of the tarsi and toes of birds have great
influence on their mode of life and habits and are often used
as specific or even generic characters, I have prepared a
diagram (Fig. 7) to show the variation in these parts only, among
twenty specimens of each of four species of birds, four or five of
the most variable alone being given. The extreme divergence
of each of the lines in a vertical direction shows the actual
amount of variation ; and if we consider the small length of
the toes of these small birds, averaging about three-quarters of
an inch, we shall see that the variation is really very large ;
while the diverging curves and angles show that each part
varies, to a great extent, independently. It is evident that
if we compared some thousands of individuals instead of
only twenty, we should have an amount of independent
variation occurring each year which would enable almost any
modification of these important organs to be rapidly effected.

In order to meet the objection that the large amount of
variability here shown depends chiefly on the observations
of one person and on the birds of a single country, I have
examined Professor Schlegel's Catalogue of the Birds in the
Leyclen Museum, in which he usually gives the range of
variation of the specimens in the museum (which are
commonly less than a dozen and rarely over twenty) as
regards some of their more important dimensions. These
fully support the statement of Mr. Allen, since they show an
equal amount of variability when the numbers compared are

60

DARWINISM

12 3 4

From Table G. io Allen's Birds of Florida.

Fig. 7.— Variation of Tarsus and Toes.

DIAGRAM OF VARIATION

61

Phonygama atra

I-.

K °a. S

I ! i

Orio/us galbula

Pica caudata

Semeioptera wallacei

Pyrrhocorax alpinus

1

l.i

I

Pig. 8. — Variation of Birds in Leyden Museum.

62 DARWINISM

sufficient, which, however, is not often the case. The
accompanying diagram exhibits the actual differences of size
in five organs which occur in five species taken almost at
random from this catalogue. Here, again, we perceive that
the variation is decidedly large, even among a very small
number of specimens ; while the facts all show that there is
no ground whatever for the common assumption that natural
species consist of individuals which are nearly all alike, or
that the variations which occur are " infinitesimal " or even
"small."

The proportionate Number of Individuals which present a
considerable amount of Variation.

The notion that variation is a comparatively exceptional
phenomenon, and that in any case considerable variations
occur very rarely in proportion to the number of individuals
which do not vary, is so deeply rooted that it is necessary to
show by every possible method of illustration how completely
opposed it is to the facts of nature. I have therefore
prepared some diagrams in which each of the individual birds
measured is represented by a spot, placed at a proportionate
distance, right and left, from the median line accordingly as
it varies in excess or defect of the mean length as regards the
particular part compared. As the object in this set of dia-
grams is to show the number of individuals which vary con-
siderably in proportion to those which vary little or not at
all, the scale has been enlarged in order to allow room for
placing the spots without overlapping each other.

In the diagram opposite twenty males of Icterus Baltimore
are registered, so as to exhibit to the eye the proportionate
number of specimens which vary, to a greater or less amount,
in the length of the tail, wing, tarsus, middle toe, hind toe, and
bill. It will be noticed that there is usually no Arery great
accumulation of dots about the median line which shows the
average dimensions, but that a considerable number are spread
at varying distances on each side of it.

In the next diagram (Fig. 10), showing the variation
among forty males of Agelseus phoeniceus, this approach to an
equable spreading of the variations is still more apparent ;
while in Fig. 12, where fifty- eight specimens of Cardinalis

in VARIABILITY OF SPECIES IN A STATE OF NATURE 63

virginianus are registered, we see a remarkable spreading out
of the spots, showing in some of the characters a tendency to
segregation into two or more groups of individuals, each vary-
ing considerably from the mean.

In order fully to appreciate the teaching of these diagrams,

VARIATION OF

ICTERUS BALTIMORE. 20.<?

Tai

9 9 9

• o 9

.ul. © 9
• • • • • •

e • ® • • 9 •

Wing.

© 3 9 © ® © ©

e © & © © © © ©

Tarsus

o

© ©9
© • O •©««

Middle Toe.

9 99 9 9

9999 9 • 9 9

Hind

99
99999 9

Toe.

9

99
9 9©© 9 _
9 9999*9999

Bill,

9^
©9

99©
9 999

Bill,

999 9

©99 99

Length.

9
999999

Width.

9
99

• 9

9® 9

Fig. 9.

we must remember, that, whatever kind and amount of varia-
tions are exhibited by the few specimens here compared,
would be greatly extended and brought into symmetrica]
form if lara;e numbers — thousands or millions — were sub-
jected to the same process of measurement and registration.
We know, from the general law which governs variations
from a mean value, that with increasing numbers the range

64

DARWINISM

VARIATION

of 40 MALES of

AGEL£US RHCE/VICEUS.

Length of Bill.

© ®

o 0000© ©00

o ©•••• ©•©••©•© ©

Total Length of Bird.

ooo

00
00©

<3

• 0

9©

09O© OOOO09ttO©9G©O© ©

Length

9 ©9 ©O©

of Tail.

»

99

© 999
9© 9999

•999©©090©©Ott©099999

Length of Wing.

© e ®99

O 999999 9
99 999999 99 9
0O0909O©©©©©©©© 9

Amount of
BILL, jj
TAIL. 1

Variation.
LENGTH |
WING. 4-

Fig. 10.

of variation of each part would increase also, at first rather
rapidly and then more slowly ■ while gaps and irregularities

Curves of Variation
Fig. 11.

would he gradually filled up, and at length the distribution
of the dots would indicate a tolerably regular curve of double
curvature like those shown in Fig. 11. The great divergence

in VARIABILITY OF SPECIES IN A STATE OF NATURE 65

of the dots, when even a few specimens are compared, shows
that the curve, with high numbers, would be a flat one like the
lower curve in the illustration here given. This being the case it
would follow that a very large proportion of the total number of
individuals constituting a species would diverge considerably
from its average condition as regards each part or organ ; and
as we know from the previous diagrams of variation (Figs. 1
to 7) that each part varies to a considerable extent, inde-
pendently, the materials constantly ready for natural selection

GARDINAUS VIRGINIAlWS. 58 specimens. Florida.

0

9

OO

Tail.

9

9

OO
©@ 9 0 0 99
000*0

o •

00 00 0

9OOOO0 OOO
00000000000000000 0

Length

o*

0O

0*0

OO***OOO0O

Wing.

of Bird.
oo

000

oooo

oooeo ••

•*•*•*•• •••

••••••*• 000

oo o

00
OOO
0000 O
OOOO 9

OOO OO
O00OOO ©

oooooooo

0 O

• Oil

e oo ooo

0O0OOOOOO

(From Allen's Birds of Florida, p. 281)

Fig. 12

to act upon are abundant in quantity and very varied in kind.
Almost any combination of variations of distinct parts will be
available, where required ; and this, as we shall see further
on, obviates one of the most weighty objections which have
been urged against the efficiency of natural selection in pro-
ducing new species, genera, and higher groups.

Variation in the Mammalia.
Owing to the generally large size of this class of animals,
and the comparatively small number of naturalists who study
them, large series of specimens are only occasionally examined

F

66 DARWINISM

and compared, and thus the materials for determining the
question of their variability in a state of nature are compara-
tively scanty. The fact that our domestic animals belonging
to this group, especially dogs, present extreme varieties not
surpassed even by pigeons and poultry among birds, renders it
almost certain that an equal amount of variability exists in the
wild state ; and this is confirmed by the example of a species of
squirrel (Sciurus carolinensis), of which sixteen specimens, all
males and all taken in Florida, were measured and tabulated
by Mr. Allen. The diagram here given shows, that, both the
general amount of the variation and the independent variability
of the several members of the body, accord completely with
the variations so common in the class of birds ; while their
amount and their independence of each other are even greater
than usual.

Variation in the Internal Organs of Animals.

In case it should be objected that the cases of variation
hitherto adduced are in the external parts only, and that
there is no proof that the internal organs vary in the same
manner, it will be advisable to show that such varieties also
occur. It is, however, impossible to adduce the same amount
of evidence in this class of variation, because the great labour
of dissecting large numbers of specimens of the same species
is rarely undertaken, and we have to trust to the chance
observations of anatomists recorded in their regular course of
study.

It must, however, be noted that a very large proportion of
the variations already recorded in the external parts of
animals necessarily imply corresponding internal variations.
When feet and .legs vary in size, it is because the bones vary ;
when the head, body, limbs, and tail change their proportions,
the bony skeleton must also change ; and even when the wing
or tail feathers of birds become longer or more numerous,
there is sure to be a corresponding change in the bones which
support and the muscles which move them. I will, however,
give a few cases of variations which have been directly
observed.

Mr. Frank E. Beddard has kindly communicated to me
some remarkable variations he has observed in the internal

DIAGRAM OF VARIATION

67

15 10 15 20 25 30 82

15 10 15 20 25 SO 32

Fia. 13.— Sciurus earolinensis. 32 specimens. Florida.

DARWINISM

organs of a species of earthworm (Perionyx excavatus). The
normal characters of this species are —

Seta? forming a complete row round each segment.

Two pairs of spermathecse — spherical pouches without

diverticula? — in segments 8 and 9.
Two pairs of testes in segments 1 1 and 1 2.
Ovaries, a single pair in segment 13.
Oviducts open by a common pore in the middle of

segment 14.
Yasa deferentia open separately in segment 18, each

furnished at its termination with a large prostate

gland.

Between two and three hundred specimens were examined,
and among them thirteen specimens exhibited the following
marked variations : —

(1) The number of the spermathecse varied from two to

three or four pairs, their position also varying.

(2) There were occasionally two pairs of ovaries, each

with its own oviduct ; the external apertures of
these varied in position, being upon segments 13
and 14, 14 and 15, or 15 and 16. Occasionally
when there was only the normal single oviduct
pore present it varied in position, once occurring on
the 10th, and once on the 11th segment.

(3) The male generative pores varied in position from

segments 14 to 20. In one instance there were two

pairs instead of the normal single pair, and in this

case each of the four apertures had its own
prostate gland.

Mr. Beddard remarks that all, or nearly all, the above
variations are found normally in other genera and species.

When Ave consider the enormous number of earthworms
and the comparatively very small number of individuals ex-
amined, we may be sure, not only that such variations as these
occur with considerable frequency, but also that still more
extraordinary deviations from the normal structure may often
exist.

The next example is taken from Mr. Darwin's unpublished
MSS.

in VARIABILITY OF SPECIES IN A STATE OF NATURE 69

" In some species of Shrews (Sorex) and in some field-mice
(Arvicola), the Rev. L. Jenyns {Ann. Nat. Hist., vol. vii. pp. 267,
272) found the proportional length of the intestinal canal to
vary considerably. He found the same variability in the
number of the caudal vertebrae. In three specimens of an
Arvicola he found the gall-bladder having a very different
degree of development, and there is reason to believe it is
sometimes absent. Professor Owen has shown that this is
the case with the gall-bladder of the giraffe."

Dr. Crisp (Proc. Zool. Soc, 1862, p. 137) found the gall-
bladder present in some specimens of Cervus superciliaris while
absent in others ; and he found it to be absent in three
giraffes which he dissected. A double gall-bladder was
found in a sheep, and in a small mammal preserved in the
Hunterian Museum there are three distinct gall-bladders.

The length of the alimentary canal varies greatly. In three
adult giraffes described by Professor Owen it was from 124 to
136 feet long; one dissected in France had this canal 211
feet long ; while Dr. Crisp measured one of the extraordinary
length of 254 feet, and similar variations are recorded in
other animals.1

The number of ribs varies in many animals. Mr. St. George
Mivart says : " In the highest forms of the Primates, the
number of true ribs is seven, but in Hylobates there are some-
times eight pairs. In Semnopithecus and Colobus there are
generally seven, but sometimes eight pairs of true ribs. In
the Cebidse there are generally seven or eight pairs, but in
Ateles sometimes nine" (Proc. Zool. Soc, 1865, p. 568). In
the same paper it is stated that the number of dorsal vertebrae
in man is normally twelve, very rarely thirteen. In the
Chimpanzee there are normally thirteen dorsal vertebra?, but
occasionally there are fourteen or only twelve.

Variations in the Skull.

Among the nine adult male Orang-utans, collected by
myself in Borneo, the skulls differed remarkably in size and
proportions. The orbits varied in width and height, the
cranial ridge was either single or double, either much or little
developed, and the zygomatic aperture varied considerably in
1 Proc. Zool. Soc, 1864, p. 64.

70

DARWINISM

2 3 4 5 6 7 8 9

Fig. 14.— Variation of Skull of Wolf. 10 specimens.

10

in VARIABILITY OF SPECIES IN A STATE OF NATURE 71

size. I noted particularly that these variations bore no
necessary relation to each other, so that a large temporal
muscle and zygomatic aperture might exist either with a
large or a small cranium ; and thus was explained the curious
difference between the single-crested and the double-crested
skulls, which had been supposed to characterise distinct species.
As an instance of the amount of variation in the skulls of
fully adult male orangs, I found the width between the orbits
externally to be only 4 inches in one specimen and fully
5 inches in another.

Exact measurements of large series of comparable skulls of
the mammalia are not easily found, but from those available
I have prepared three diagrams (Figs. 14, 15, and 16), in order
to exhibit the facts of variation in this very important organ.
The first shows the variation in ten specimens of the common
wolf (Canis lupus) from one district in North America, and
we see that it is not only large in amount, but that each
part exhibits a considerable independent variability.1

In Diagram 15 we have the variations of eight skulls of
the Indian Honey-bear (Ursus labiatus), as tabulated by the
late Dr. J. E. Gray of the British Museum. For such a
small number of specimens the amount of variation is very
large — from one-eighth to one-fifth of the mean size, — while
there are an extraordinary number of instances of inde-
pendent variability. In Diagram 16 we have the length and
width of twelve skulls of adult males of the Indian wild boar
(Sus cristatus), also given by Dr. Gray, exhibiting in both sets
of measurements a variation of more than one-sixth, combined
with a very considerable amount of independent variability.2

The few facts now given, as to variations of the internal
parts of animals, might be multiplied indefinitely by a search
through the voluminous writings of comparative anatomists.
But the evidence already adduced, taken in conjunction with
the much fuller evidence of variation in all external organs,
leads us to the conclusion that wherever variations are looked
for among a considerable number of individuals of the more

1 J. A. Allen, on Geographical Variation among North American Mammals,
Bull. U. S. Geol. and Geog. Survey, vol. ii. p. 314 (1876).

2 Proc. Zool. Soc. Lond., 1864, p. 700, and 1868, p. 28.

72

DARWINISM

/ 2 3 4 5 6 7

(From Table by Dr.J.E. Gray. P.Z.S. 1864. pJQO.)

Fig. 15.— Variation of 8 skulls (Ursus labiatus).

DIAGRAM OF VARIATION

73

74 DARWINISM

common species they are sure to be found ; that they are
everywhere of considerable amount, often reaching 20 per
cent of the size of the j)art implicated ; and that they are to
a great extent independent of each other, and thus afford
almost any combination of variations that may be needed.

It must be particularly noticed that the whole series of
variation-diagrams here given (except the three which illustrate
the number of varying individuals) in every case represent the
actual amount of the variation, not on any reduced or enlarged
scale, but as it were life-size. Whatever number of inches or
decimals of an inch the species varies in any of its parts is
marked on the diagrams, so that with the help of an ordinary
divided rule or a pair of compasses the variation of the
different parts can be ascertained and compared just as if the
specimens themselves were before the reader, but with much
greater ease.

In my lectures on the Darwinian theory in America and
in this country I used diagrams constructed on a different
plan, equally illustrating the large amount of independent
variability, but less simple and less intelligible. The present
method is a modification of that used by Mr. Francis Galton
in his researches on the theory of variability, the upper line
(showing the variability of the body) in Diagrams 4, 5, 6, and
13, being laid down on the method he has used in his experi-
ments with sweet-peas and in pedigree moth-breeding.1 I be-
lieve, after much consideration, and many tedious experiments
in diagram-making, that no better method can be adopted for
bringing before the eye, both the amount and the peculiar
features of individual variability.

Variations of the Habits of Animals.

Closely connected with those variations of internal and
external structure which have been already described, are the
changes of habits which often occur in certain individuals or
in whole species, since these must necessarily depend upon some
corresponding change in the brain or in other parts of the
organism ; and as these changes are of great importance in
relation to the theory of instinct, a few examples of them will
be now adduced.

1 See Trans. Entomological Society of London, 1887, p. 24.

in VARIABILITY OF SPECIES IN A STATE OF NATURE 75

The Kea (Nestor notabilis) is a curious parrot inhabiting the
mountain ranges of the Middle Island of New Zealand. It
belongs to the family of Brush-tongued parrots, and naturally
feeds on the honey of flowers and the insects which frequent
them, together with such fruits or berries as are found in the
region. Till quite recently this comprised its whole diet, but
since the country it inhabits has become occupied by Europeans
it has developed a taste for a carnivorous diet, with alarming
results. It began by picking the sheepskins hung out to dry
or the meat in process of being cured. About 1868 it was
first observed to attack living sheep, which had frequently
been found with raw and bleeding wounds on their backs.
Since then it is stated that the bird actually burrows into the
living sheep, eating its way down to the kidneys, which form
its special delicacy. As a natural consequence, the bird is
being destroyed as rapidly as possible, and one of the rare
and curious members of the New Zealand fauna will no
doubt shortly cease to exist. The case affords a remark-
able instance of how the climbing feet and powerful hooked
beak developed for one set of purposes can be applied to
another altogether different purpose, and it also shows how
little real stability there may be in what appear to us the
most fixed habits of life. A somewhat similar change of diet
has been recorded by the Duke of Argyll, in which a goose,
reared by a golden eagle, was taught by its foster -parent to
eat flesh, which it continued to do regularly and apparently
with great relish.1

Change of habits appears to be often a result of imitation,
of which Mr. Tegetmeier gives some good examples. He states
that if pigeons are reared exclusively with small grain, as
wheat or barley, they will starve before eating beans. But
when they are thus starving, if a bean-eating pigeon is put
among them, they follow its example, and thereafter adopt
the habit. So fowls sometimes refuse to eat maize, but on
seeing others eat it, they do the same and become excessively
fond of it. Many persons have found that their yellow
crocuses were eaten by sparrows, while the blue, purple, and
white coloured varieties were left untouched ; but Mr. Teget-
meier, who grows only these latter colours, found that after
1 Nature, vol. xix. p. 554.

DARWINISM

two years the sparrows began to attack them, and thereafter
destroyed them quite as readily as the yellow ones ; and he
believes it was merely because some bolder sparrow than the
rest set the example. On this subject Mr. Charles C. Abbott
well remarks : "In studying the habits of our American birds
— and I suppose it is true of birds everywhere — it must at all
times be remembered that there is less stability in the habits
of birds than is usually supposed ; and no account of the habits
of any one species will exactly detail the various features of
its habits as they really are, in every portion of the terri-
tory it inhabits." l

Mr. Charles Dixon has recorded a remarkable change in the
mode of nest-building of some common chaffinches which were
taken to New Zealand and turned out there. He says : " The
cup of the nest is small, loosely put together, apparently lined
with feathers, and the walls of the structure are prolonged for
about 18 inches, and hang loosely down the side of the
supporting branch. The whole structure bears some re-
semblance to the nests of the hangnests (Icteridse), with the
exception that the cavity is at the top. Clearly these New
Zealand chaffinches were at a loss for a design when fabricat-
ing their nest. They had no standard to work by, no nests of
their own kind to copy, no older birds to give them any instruc-
tion, and the result is the abnormal structure I have just
described." 2

These few examples are sufficient to show that both the
habits and instincts of animals are subject to variation ; and
had we a sufficient number of detailed observations we should
probably find that these variations were as numerous, as
diverse in character, as large in amount, and as independent
of each other as those which we have seen to characterise
their bodily structure.

The Variability of Plants.

The variability of plants is notorious, being proved not only
by the endless variations which occur whenever a species is
largely grown by horticulturists, but also by the great difficulty
that is felt by botanists in determining the limits of species in

1 Nature, vol. xvi. p. 163 ; ancl vol. xi. p. 227.
2 Ibid., vol. xxxi. (1885), p. 533.

in VARIABILITY OF SPECIES IN A STATE OF NATURE 77

many large genera. As examples we may take the roses, the
brambles, and the willows as well illustrating this fact. In Mr.
Baker's Revision of the British Roses (published by the Linnean
Society in 1863), he includes under the single species, Rosa
canina — the common dog-rose — no less than twenty -eight
named varieties distinguished by more or less constant characters
and often confined to special localities, and to these are
referred about seventy of the species of British and continental
botanists. Of the genus Rubus or bramble, five British species
are given in Bentham's Handbook of the British Flora, while
in the fifth edition of Babington's Manual of British Botany,
published about the same time, no less than forhj-five species
are described. Of willows (Salix) the same two works
enumerate fifteen and thirty -one species respectively. The
hawkweeds (Hieracium) are equally puzzling, for while Mr.
Bentham admits only seven British species, Professor Babing-
ton describes no less than thirty-two, besides several named
varieties.

A French botanist, Mons. A. Jordan, has collected numerous
forms of a common little plant, the spring whitlow- grass
(Draba verna) ; he has cultivated these for several successive
years, and declares that they preserve their peculiarities un-
changed ; he also says that they each come true from seed,
and thus possess all the characteristics of true species. He
has described no less than fifty-two such species or permanent
varieties, all found in the south of France ; and he urges
botanists to follow his example in collecting, describing, and
cultivating all such varieties as may occur in their respective
districts. Now, as the plant is very common almost all over
Europe and ranges from North America to the Himalayas,
the number of similar forms over this wide area would prob-
ably have to be reckoned by hundreds if not by thousands.

The class of facts now adduced must certainly be held
to prove that in many large genera and in some single species
there is a very large amount of variation, which renders it
quite impossible for experts to agree upon the limits of species.
We will now adduce a few striking cases of individual
variation.

The distinguished botanist, Alp. de Candolle, made a special
study of the oaks of the whole world, and has stated some

DARWINISM

remarkable facts as to their variability. He declares that on
the same branch of oak he has noted the following variations :
(1) In the length of the petiole, as one to three ; (2) in the form
of the leaf, being either elliptical or obovoid ; (3) in the margin
being entire, or notched, or even pinnatifid ; (4) in the ex-
tremity being acute or blunt ; (5) in the base being sharp,
blunt, or cordate ; (6) in the surface being pubescent or
smooth ; (7) the perianth varies in depth and lobing ; (8)
the stamens vary in number, independently ; (9) the anthers
are mucronate or blunt; (10) the fruit stalks vary greatly
in length, often as one to three; (11) the number of fruits
varies ; (12) the form of the base of the cup varies ; (13) the
scales of the cup vary in form; (14) the proportions of the
acorns vary ; (15) the times of the acorns ripening and falling
vary.

Besides this, many species exhibit well-marked varieties
which have been described and named, and these are most
numerous in the best-known species. Our British oak (Quercus
robur) has twenty -eight varieties; Quercus Lusitanica has
eleven ; Quercus calliprinos has ten ; and Quercus coccif era
eight.

A most remarkable case of variation in the parts of a
common flower has been given by Dr. Hermann Miiller. He
examined two hundred flowers of Myosurus minimus, among
which he found thirty-five different proportions of the sepals,
petals, and anthers, the first varying from four to seven, the
second from two to five, and the third from two to ten. Five
sepals occurred in one hundred and eighty-nine out of the two
hundred, but of these one hundred and five had three petals,
forty-six had four petals, and twenty-six had five petals ; but
in each of these sets the anthers varied in number from three
to eight, or from two to nine. We have here an example of
the same amount of "independent variability" that, as we
have seen, occurs in the various dimensions of birds and
mammals ; and it may be taken as an illustration of the kind
and degree of variability that may be expected to occur
among small and little specialised flowers.1

In the common wind-flower (Anemone nemorosa) an almost
equal amount of variation occurs ; and I have myself gathered
1 Nature, vol. xxvi. p. 81.

m VARIABILITY OF SPECIES IN A STATE OF NATURE 79

in one locality flowers varying from | inch to If inch in
diameter ; the bracts varying from 1| inch to 4 inches across;
and the petaloid sepals either broad or narrow, and varying
in number from five to ten. Though generally pure white
on their upper surface, some specimens are a full pink, while
others have a decided bluish tinge.

Mr. Darwin states that he carefully examined a large number
of plants of Geranium phseum and G. pyrenaicum (not perhaps
truly British but frequently found wild), which had escaped
from cultivation, and had spread by seed in an open planta-
tion ; and he declares that " the seedlings varied in almost
every single character, both in their flowers and foliage, to a
degree which I have never seen exceeded ; yet they could not
have been exposed to any great change of their conditions."1

The following examples of variation in important parts of
plants were collected by Mr. Darwin and have been copied
from his unpublished MSS. : —

" De Candolle (Mem. Soc. Phys. de Geneve, torn. ii. part ii.
p. 217) states that Papaver bracteatum and P. orientale present
•indifferently two sepals and four petals, or three sepals and
six petals, which is sufficiently rare with other species of the
genus."

" In the Primulacese and in the great class to which this
family belongs the unilocular ovarium is free, but M. Dubury
(Mem. Soc. Phys. de Geneve, torn. ii. p. 406) has often found
individuals in Cyclamen hedersefolium, in which the base of
the ovary was connected for a third part of its length with
the inferior part of the calyx."

"M. Aug. St. Hilaire (Sur la Gynobase, Mem. des Mus.
d'Hist. Nat, torn. x. p. 134), speaking of some bushes of the
Gomphia olesefolia, which he at first thought formed a quite
distinct species, says : ' Voila done dans un meme individu
des loges et un style qui se rattachent tantot a un axe vertical,
et tantot a un gynobase ; done celui-ci n'est qu'un axe veri-
table ; mais cet axe est deprime au lieu d'etre vertical." He
adds (p. 151), 'Does not all this indicate that nature has
tried, in a manner, in the family of Rutaceae to produce from
a single multilocular ovary, one-styled and symmetrical,
several unilocular ovaries, each with its own style.' And he

1 Animals and Plants under Domestication, vol. ii. p. 258.

80 DARWINISM

subsequently shows that, in Xanthoxylum monogynum, ' it
often happens that on the same plant, on the same panicle,
we find flowers with one or with two ovaries ;' and that this is
an important character is shown by the Rutacese (to which
Xanthoxylum belongs), being placed in a group of natural
orders characterised by having a solitary ovary."

" De Candolle has divided the Cruciferae into five sub-orders
in accordance with the position of the radicle and cotyledons,
yet Mons. T. G-ay (Ann. des Scien. Nat, ser. i. torn. vii. p. 389)
found in sixteen seeds of Petrocallis Pyrenaica the form of the
embryo so uncertain that he could not tell whether it ought
to be placed in the sub-orders ' Pleurorhiz6e ' or ' Notorhizee ' ;
so again (p. 400) in Cochlearia saxatilis M. Gay examined
twenty-nine embryos, and of these sixteen were vigorously
' pleurorhiz6es,' nine had characters intermediate between
pleuro- and notor- hiz6es, and four were pure notorhizees."

"M. Easpail asserts (Ann. des Scien. Nat., ser. i. torn. v. p.
440) that a grass (Nostus Borbonicus) is so eminently variable
in its floral organisation, that the varieties might serve to
make a family with sufficiently numerous genera and tribes —
a remark which shows that important organs must be here

variable."

Species which vary little.

The preceding statements, as to the great amount of
variation occurring in animals and plants, do not prove
that all species vary to the same extent, or even vary at
all, but, merely, that a considerable number of species in
every class, order, and family do so vary. It will have
been observed that the examples of great variability have
all been taken from common species, or species which have
a wide range and are abundant in individuals. Now Mr.
Darwin concludes, from an elaborate examination of the floras
and faunas of several distinct regions, that common, wide
ranging species, as a rule, vary most, while those that are
confined to special districts and are therefore comparatively
limited in number of individuals vary least. By a similar
comparison it is shown that species of large genera vary more
than species of small genera. These facts explain, to some
extent, why the opinion has been so prevalent that variation
is very limited in amount and exceptional in character. For

ill VARIABILITY OF SPECIES IN A STATE OF NATURE 81

naturalists of the old school, and all mere collectors, were
interested in species in proportion to their rarity, and would
often have in their collections a larger number of specimens
of a rare species than of a species that was very common.
Now as these rare species do really vary much less than the
common species, and in many cases hardly vary at all, it was
very natural that a belief in the fixity of species should
prevail. It is not, however, as we shall see presently, the
rare, but the common and widespread species which become
the parents of new forms, and thus the non-variability of any
number of rare or local species offers no difficulty whatever in
the way of the theory of evolution.

Concluding Remarks.

We have now shown in some detail, at the risk of being
tedious, that individual variability is a general character of all
common and widespread species of animals or plants ; and,
further, that this variability extends, so far as we know, to
every part and organ, whether external or internal, as well as
to every mental faculty. Yet more important is the fact that
each part or organ varies to a considerable extent inde-
pendently of other parts. Again, we have shown, by abundant
evidence, that the variation that occurs is very large in
amount — usually reaching 10 or 20, and sometimes even 25
per cent of the average size of the varying part; while
not one or two only, but from 5 to 10 per cent of the speci-
mens examined exhibit nearly as large an amount of variation.
These facts have been brought clearly before the reader by
means of numerous diagrams, drawn to scale and exhibiting
the actual variations in inches, so that there can be no pos-
sibility of denying either their generality or their amount.
The importance of this full exposition of the subject will be
seen in future chapters, when we shall frequently have to
refer to the facts here set forth, especially when we deal with
the various theories of recent writers and the criticisms that
have been made of the Darwinian theory.

A full exposition of the facts of variation among wild
animals and plants is the more necessary, because compara-
tively few of them were published in Mr. Darwin's works,
while the more important have only been made known since

G

82 DARWINISM

the last edition of The Origin of Species was prepared ; and it
is clear that Mr. Darwin himself did not fully recognise the
enormous amount of variability that actually exists. This
is indicated by his frequent reference to the extreme slowness
of the changes for which variation furnishes the materials,
and also by his use of such expressions as the following : "A
variety when once formed must again, perhaps after a long
interval of time, vary or present individual differences of the
same favourable nature as before " (Origin, p. 66). And
again, after speaking of changed conditions " affording a better
chance of the occurrence of favourable variations," he adds :
" Unless such occur natural selection can do nothing" (Origin,
p. 64). These expressions are hardly consistent with the
fact of the constant and large amount of variation, of every
part, in all directions, which evidently occurs in each genera-
tion of all the more abundant species, and which must afford
an ample supply of favourable variations whenever required ;
and they have been seized upon and exaggerated by some
writers as proofs of the extreme difficulties in the way of the
theory. It is to show that such difficulties do not exist, and
in the full conviction that an adequate knowledge of the
facts of variation affords the only sure foundation for the
Darwinian theory of the origin of species, that this chapter
has been written.

CHAPTER IV

VARIATION OF DOMESTICATED ANIMALS AND
CULTIVATED PLANTS

The facts of variation and artificial selection — Proofs of the generality of
variation — Variations of apples and melons — Variations of flowers —
Variations of domestic animals — Domestic pigeons — Acclimatisation
— Circumstances favourable to selection by man — Conditions favour-
able to variation — Concluding remarks.

Having so fully discussed variation under nature it will be
unnecessary to devote so much space to domesticated animals
and cultivated plants, especially as Mr. Darwin has published
two remarkable volumes on the subject where those who
desire it may obtain ample information. A general sketch of
the more important facts will, however, be given, for the
purpose of showing how closely they correspond with those
described in the preceding chapter, and also to point out the
general principles which they illustrate. It will also be
necessary to explain how these variations have been increased
and accumulated by artificial selection, since we are thereby
better enabled to understand the action of natural selection, to
be discussed in the succeeding chapter.

The facts of Variation and Artificial Selection.

Every one knows that in each litter of kittens or of
puppies no two are alike. Even in the case in which several
are exactly alike in colours, other differences are always
perceptible to those who observe them closely. They will
differ in size, in the proportions of their bodies and limbs, in
the length or texture of their hairy covering, and notably
in their disposition. They each possess, too, an individual

84 DARWINISM

countenance, almost as varied when closely studied as that of
a human being; not only can a shepherd distinguish every
sheep in his flock, but we all know that each kitten in the
successive families of our old favourite cat has a face of its
own, with an expression and individuality distinct from all
its brothers and sisters. Now this individual variability
exists among all creatures whatever, which we can closely
observe, even when the two parents are very much alike and
have been matched in order to preserve some special breed.
The same thing occurs in the vegetable kingdom. All plants
raised from seed differ more or less from each other. In
every bed of flowers or of vegetables we shall find, if we look
closely, that there are countless small differences, in the size,
in the mode of growth, in the shape or colour of the leaves,
in the form, colour, or markings of the flowers, or in the size,
form, colour, or flavour of the fruit. These differences are
usually small, but are yet easily seen, and in their extremes
are very considerable ; and they have this important quality,
that they have a tendency to be reproduced, and thus by
careful breeding any particular variation or group of varia-
tions can be increased to an enormous extent — apparently to
any extent not incompatible with the life, growth, and re-
production of the plant or animal.

The way this is done is by artificial selection, and it is
very important to understand this process and its results.
Suppose we have a plant with a small edible seed, and we
want to increase the size of that seed. We grow as large a
quantity of it as possible, and when the crop is ripe we
carefully choose a few of the very largest seeds, or we may
by means of a sieve sort out a quantity of the largest seeds.
Next year we sow only these large seeds, taking care to give
them suitable soil and manure, and the result is found to be
that the average size of the seeds is larger than in the first
crop, and that the largest seeds are now somewhat larger and
more numerous. Again sowing these, we obtain a further
slight increase of size, and in a very few years we obtain a
greatly improved race, which will always produce larger seeds
than the unimproved race, even if cultivated Avithout any
special care. In this way all our fine sorts of vegetables,
fruits, and flowers have been obtained, all our choice breeds

iv VARIATION UNDER DOMESTICATION 85

of cattle or of poultry, our wonderful race-horses, and our
endless varieties of dogs. It is a very common but mistaken
idea that this improvement is due to crossing and feeding in
the case of animals, and to improved cultivation in the case
of plants. Crossing is occasionally used in order to obtain a
combination of qualities found in two distinct breeds, and
also because it is found to increase the constitutional vigour ;
but every breed possessing any exceptional quality is the
result of the selection of variations occurring year after year
and accumulated in the manner just described. Purity of
breed, with rejDeated selection of the best varieties of that
breed, is the foundation of all improvement in our domestic
animals and cultivated plants.

Proofs of the Generality of Variation.

Another very common error is, that variation is the
exception, and rather a rare exception, and that it occurs
only in one direction at a time — that is, that only one or two
of the numerous possible modes of variation occur at the same
time. The experience of breeders and cultivators, however,
proves that variation is the rule instead of the exception, and
that it occurs, more or less, in almost every direction. This is
shown by the fact that different species of plants and animals
have required different hinds of modification to adapt them to
our use, and we have never failed to meet with variation in
that particular direction, so as to enable us to accumulate it and
so to produce ultimately a large amount of change in the
required direction. Our gardens furnish us with numberless
examples of this property of plants. In the cabbage and
lettuce we have found variation in the size and mode of
growth of the leaf, enabling us to produce by selection the
almost innumerable varieties, some with solid heads of foliage
quite unlike any plant in a state of nature, others with
curiously wrinkled leaves like the savoy, others of a deep
purple colour used for pickling. From the very same species
as the cabbage (Brassica oleracea) have arisen the broccoli
and cauliflower, in which the leaves have undergone little
alteration, while the branching heads of flowers grow into a
compact mass forming one of our most delicate vegetables.
The brussels sprouts are another form of the same plant, in

DARWINISM

which the whole mode of growth has been altered, numerous
little heads of leaves being produced on the stem. In other
varieties the ribs of the leaves are thickened so as to become
themselves a culinary vegetable ; while, in the Kohlrabi, the
stem grows into a turnip-like mass just above ground. Now
all these extraordinarily distinct plants come from one original
species which still grows wild on our coasts ; and it must have
varied in all these directions, otherwise variations could not
have been accumulated to the extent we now see them. The
flowers and seeds of all these plants have remained nearly
stationary, because no attempt has been made to accumulate
the slight variations that no doubt occur in them.

If now we turn to another set of plants, the turnips,
radishes, carrots, and potatoes, we find that the roots or under-
ground tubers have been wonderfully enlarged and improved,
and also altered in shape and colour, while the stems, leaves,
flowers, and fruits have remained almost unchanged. In the
various kinds of peas and beans it is the pod or fruit and the
seed that has been subjected to selection, and therefore greatly
modified ; and it is here very important to notice that while
all these plants have undergone cultivation in a great variety
of soils and climates, with different manures and under
different systems, yet the flowers have remained but little
altered, those of the broad bean, the scarlet-runner, and the
garden-pea, being nearly the same in all the varieties. This
shows us how little change is produced by mere cultivation,
or even by variety of soil and climate, if there is no selection
to preserve and accumulate the small variations that are con-
tinually occurring. When, however, a great amount of modifi-
cation has been effected in one country, change to another
country produces a decided effect. Thus it has been found
that some of the numerous varieties of maize produced and
cultivated in the United States change considerably, not only
in their size and colour, but even in the shape of the seed when
grown for a few successive years in Germany.1 In all our
cultivated fruit trees the fruits vary immensely in shape, size,
colour, flavour, time of ripening, and other qualities, while the
leaves and flowers usually differ so little that they are hardly
distinguishable except to a very close observer.

1 Darwin, Animals and Plants under Domestication, vol. i. p. 322.

VARIATION UNDER DOMESTICATION 87

Variations of Apples and of Melons.

The most remarkable varieties are afforded by the apple
and the melon, and some account of these will be given as
illustrating the effects of slight variations accumulated by
selection. All our apples are known to have descended from
the common crab of our hedges (Pyrus malus), and from this
at least a thousand distinct varieties have been produced.
These differ greatly in the size and form of the fruit, in its
colour, and in the texture of the skin. They further differ in
the time of ripening, in their flavour, and in their keeping
properties ; but apple trees also differ in many other ways.
The foliage of the different varieties can often be distinguished
by peculiarities of form and colour, and it varies considerably
in the time of its appearance ; in some hardly a leaf appears
till the tree is in full bloom, while others produce their leaves
so early as almost to hide the flowers. The flowers differ in
size and colour, and in one case in structure also, that of the
St. Valery apple having a double calyx with ten divisions, and
fourteen styles with oblique stigmas, but without stamens or
corolla. The flowers, therefore, have to be fertilised with the
pollen from other varieties in order to produce fruit. The
pips or seeds differ also in shape, size, and colour ; some
varieties are liable to canker more than others, while the
Winter Majetin and one or two others have the strange con-
stitutional peculiarity of never being attacked by the mealy
bug even when all the other trees in the same orchard are in-
fested with it.

All the cucumbers and gourds vary immensely, but the
melon (Cucumis melo) exceeds them all. A French botanist,
M. Naudin, devoted six years to their study. He found that
previous botanists had described thirty distinct species, as they
thought, which were really only varieties of melons. They
differ chiefly in their fruits, but also very much in foliage and
mode of growth. Some melons are only as large as small
plums, others weigh as much as sixty-six pounds. One variety
has a scarlet fruit. Another is not more than an inch in
diameter, but sometimes more than a yard in length, twisting
about in all directions like a serpent. Some melons are
exactly like cucumbers ; and an Algerian variety, when ripe,

DARWINISM chap.

cracks and falls to pieces, just as occurs in a wild gourd
(C. momordica).1

Variations of Flowers.

Turning to flowers, we find that in the same genus as our
currant and gooseberry, which we have cultivated for their
fruits, there are some ornamental species, as the Ribes sanguinea,
and in these the flowers have been selected so as to produce deep
red, pink, or white varieties. When any particular flower be-
comes fashionable and is grown in large quantities, variations
are always met with sufficient to produce great varieties of tint
or marking, as shown by our roses, auriculas, and geraniums.
When varied leaves are required, it is found that a number of
plants vary sufficiently in this direction also, and we have
zonal geraniums, variegated ivies, gold and silver marked
hollies, and many others.

Variations of Domestic Animals.

Coming now to our domesticated animals, we find still more
extraordinary cases ; and it appears as if any special quality or
modification in an animal can be obtained if we only breed it
in sufficient quantity, watch carefully for the required varia-
tions, and carry on selection with patience and skill for a
sufficiently long period. Thus, in sheep we have enormously
increased the wool, and have obtained the power of rapidly
forming flesh and fat ; in cows we have increased the produc-
tion of milk ; in horses we have obtained strength, endurance,
or speed, and have greatly modified size, form, and colour ; in
poultry we have secured various colours of plumage, increase
of size, and almost perpetual egg-laying. But it is in dogs and
pigeons that the most marvellous changes have been effected,
and these require our special attention.

Our various domestic dogs are believed to have originated
from several distinct wild species, because in every part of
the world the native dogs resemble some wild dogs or wolves
of the same country. Thus perhaps several species of wolves
and jackals were domesticated in very early times, and from
breeds derived from these, crossed and improved by selection,

1 These facts are taken from Darwin's Domesticated Animals and Cultivated
Plants, vol. i. pp. 359, 360, 392-401 ; vol. ii. pp. 231, 275, 330.

iv VARIATION UNDER DOMESTICATION 89

our existing dogs have descended. But this intermixture of
distinct species will go a very little way in accounting for the
peculiarities of the different breeds of dogs, many of which are
totally unlike any wild animal. Such is the case with grey-
hounds, bloodhounds, bulldogs, Blenheim spaniels, terriers,
pugs, turnspits, pointers, and many others ; and these differ
so greatly in size, shape, colour, and habits, as well as in the
form and proportions of all the different parts of the body,
that it seems impossible that they could have descended from
any of the known wild dogs, wolves, or allied animals, none
of which differ nearly so much in size, form, and proportions.
We have here a remarkable proof that variation is not con-
fined to superficial characters — to the colour, hair, or external
appendages, when we see how the entire skeletons of such
forms as the greyhound and the bulldog have been gradually
changed in opposite directions till they are both completely
unlike that of any known wild animal, recent or extinct.
These changes have been the result of some thousands of years
of domestication and selection, different breeds being used and
preserved for different purposes ; but some of the best breeds
are known to have been improved and perfected in modern
times. About the middle of the last century a new and im-
proved kind of foxhound was produced ; the greyhound was
also greatly improved at the end of the last century, while the
true bulldog was brought to perfection about the same period.
The Newfoundland dog has been so much changed since it was
first imported that it is now quite unlike any existing native
dog in that island.1

Domestic Pigeons.

The most remarkable and instructive example of variation
produced by human selection is afforded by the various races
and breeds of domestic pigeons, not only because the varia-
tions produced are often most extraordinary in amount and
diverse in character, but because in this case there is no
doubt whatever that all have been derived from one wild
species, the common rock-pigeon (Columba livia). As this is a
very important point it is well to state the evidence on which
the belief is founded. The wild rock-pigeon is of a slaty-blue

1 See Darwin's Animals and Plants under Domestication, vol. i. pp. 40-42.

90 DARWINISM

colour, the tail has a dark band across the end, the wings
have two black bands, and the outer tail-feathers are edged
with white at the base. No other wild pigeon in the world
has this combination of characters. Now in every one of the
domestic varieties, even the most extreme, all the above
marks, even to the white edging of the outer tail-feathers,
are sometimes found perfectly developed. When birds
belonging to two distinct breeds are crossed one or more
times, neither of the parents being blue, or having any of the
above-named marks, the mongrel offspring are very apt to
acquire some of these characters. Mr. Darwin gives instances
which he observed himself. He crossed some white fantails
with some black barbs, and the mongrels were black, brown,
or mottled. He also crossed a barb with a spot, which is a
white bird with a red tail and red spot on the forehead, and
the mongrel offspring were dusky and mottled. On now
crossing these two sets of mongrels with each other, he
obtained a bird of a beautiful blue colour, with the barred
and white edged tail, and double-banded wings, so as almost
exactly to resemble a wild rock -pigeon. This bird was
descended in the second generation from a pure white and
pure black bird, both of which when unmixed breed their
kind remarkably true. ' These facts, well known to ex-
perienced pigeon -fanciers, together with the habits of the
birds, which all like to nest in holes, or dovecots, not in trees
like the great majority of wild pigeons, have led to the general
belief in the single origin of all the different kinds.

In order to afford some idea of the great differences which
exist among domesticated pigeons, it will be well to give a
brief abstract of Mr. Darwin's account of them. He divides
them into eleven distinct races, most of Avhich have several
sub-races.

Race I. Pouters. — These are especially distinguished by
the enormously enlarged crop, Avhich can be so inflated in
some birds as almost to conceal the beak. They are very long
in the body and legs and stand almost upright, so as to
present a very distinct appearance. Their skeleton has
become modified, the ribs being broader and the vertebrae
more numerous than in other pigeons.

iv VARIATION UNDER DOMESTICATION 91

Race II. Carriers. — These are large, long-necked birds,
with a long pointed beak, and the eyes surrounded with a
naked carunculated skin or wattle, which is also largely
developed at the base of the beak. The opening of the
mouth is unusually wide. There are several sub-races, one
being called Dragons.

Race III. Bunts. — These are very large-bodied, long-beaked
pigeons, with naked skin round the eyes. The wings are
usually very long, the legs long, and the feet large, and the
skin of the neck is often red. There are several sub-races,
and these differ very much, forming a series of links between
the wild rock-pigeon and the carrier.

Race IV. Barbs. — These are remarkable for their very
short and thick beak, so unlike that of most pigeons that
fanciers compare it with that of a bullfinch. They have also
a naked carunculated skin round the eyes, and the skin over
the nostrils swollen.

Race V. Fantails. — Short-bodied and rather small-beaked
pigeons, with an enormously developed tail, consisting usually
of from fourteen to forty feathers instead of twelve, the
regular number in all other pigeons, wild and tame. The
tail spreads out like a fan and is usually carried erect, and
the bird bends back its slender neck, so that in highly -bred
varieties the head touches the tail. The feet are small, and
they walk stiffly.

Race VI. Turbits and Owls. — These are characterised by
the feathers of the middle of neck and breast in front
spreading out irregularly so as to form a frill. The Turbits
also have a crest on the head, and both have the beak
exceedingly short.

Race VII. Tumblers. — These have a small body and short
beak, but they are specially distinguished by the singular
habit of tumbling over backwards during flight. One of the
sub -races, the Indian Lotan or Ground tumbler, if slightly
shaken and placed on the ground, will immediately begin
tumbling head over heels until taken up and soothed. If not
taken up, some of them will go on tumbling till they die.

92 DARWINISM

Some English tumblers are almost equally persistent. A
writer, quoted by Mr. Darwin, says that these birds generally
begin to tumble almost as soon as they can fly ; " at three
months old they tumble well, but still fly strong; at fire or
six months they tumble excessively ; and in the second year
they mostly give 'up flying, on account of their tumbling so
much and so close to the ground. Some fly round with the
flock, throwing a clean summersault every few yards till they
are obliged to settle from giddiness and exhaustion. These
are called Air- tumblers, and they commonly throw from
twenty to thirty summersaults in a minute, each clear and
clean. I have one red cock that I have on two or three
occasions timed by my watch, and counted forty summer-
saults in the minute. At first they throw a single summer-
sault, then it is double, till it becomes a continuous roll,
which puts an end to flying, for if they fly a few yards over
they go, and roll till they reach the ground. Thus I had one
kill herself, and another broke his leg. Many of them turn
over only a few inches from the ground, and will tumble two
or three times in flying across their loft. These are called
House -tumblers from tumbling in the house. The act of
tumbling seems to be one over which they have no control,
an involuntary movement which they seem to try to prevent.
I have seen a bird sometimes in his struggles fly a yard or
two straight upwards, the impulse forcing him backwards
while he struggles to go forwards." l

The Short-faced tumblers are an improved sub-race which
have almost lost the power of tumbling, but are valued for
possessing some other characteristics in an extreme degree.
They are very small, have almost globular heads, and a very
minute beak, so that fanciers say the head of a perfect bird
should resemble a cherry with a barleycorn stuck in it. Some
of these weigh less than seven ounces, whereas the wild rock-
pigeon weighs about fourteen ounces. The feet, too, are
very short and small, and the middle toe has twelve or
thirteen instead of fourteen or fifteen scutellse. They have
often only nine primary wing-feathers instead of ten as in all
other pigeons.

1 Mr. Brent in Journal of Horticulture, 1861, p. 76 ; quoted by Darwin,
Animals and Plants under Domestication, vol. i. p. 151.

iv VARIATION UNDER DOMESTICATION 93

Race VIII. Indian Frill-back. — In these birds the beak is
very short, and the feathers of the whole body are reversed
or turn backwards.

Race IX. Jacobin. — These curious birds have a hood of
feathers almost enclosing the head and meeting in front of
the neck. The wings and tail are unusually long.

Race X. Trumpeter. — Distinguished by a tuft of feathers
curling forwards over the beak, and the feet very much
feathered. They obtain their name from the peculiar voice
unlike that of any other pigeon. The coo is rapidly repeated,
and is continued for several minutes. The feet are covered
with feathers so large as often to appear like little wings.

Race XI. comprises Laughers, Frill -bach, Nuns, Spots, and
Swallows.-~^-Th.ey are all very like the common rock -pigeon,
but have each some slight peculiarity. The Laughers have a
peculiar voice, supposed to resemble a laugh. The Nuns are
white, with the head, tail, and primary wing-feathers black or
red. The Spots are white, with the tail and a spot on the
forehead red. The Swallows are slender, white in colour,
with the head and wings of some darker colour.

Besides these races and sub-races a number of other kinds
have been described, and about one hundred and fifty varieties
can be distinguished. It is interesting to note that almost
every part of the bird, whose variations can be noted and
selected, has led to variations of a considerable extent, and
many of these have necessitated changes in the plumage and
in the skeleton quite as great as any that occur in the
numerous distinct species of large genera. The form of the
skull and beak varies enormously, so that the skulls of the
Short -faced tumbler and some of the Carriers differ more
than any wild pigeons, even those classed in distinct genera.
The breadth and number of the ribs vary, as well as the
processes on them ; the number of the vertebra? and the
length of the sternum also vary ; and the perforations in the
sternum vary in size and shape. The oil gland varies in
development, and is sometimes absent. The number of the
wing-feathers varies, and those of the tail to an enormous
extent. The proportions of the leg and feet and the number

94 DARWINISM

of the scutellse also vary. The eggs also vary somewhat in
size and shape ; and the amount of downy clothing on the
young bird, when first hatched, differs very considerably.
Finally, the attitude of the body, the manner of walking, the
mode of flight, and the voice, all exhibit modifications of the
most remarkable kind.1

Acclimatisation.

A very important kind of variation is that constitutional
change termed acclimatisation, which enables any organism to
become gradually adapted to a different climate from the
parent stock. As closely allied species often inhabit different
countries possessing very different climates, we should expect
to find cases illustrating this change among our domesticated
animals and cultivated plants. A few examples will therefore
be adduced showing that such constitutional variation does
occur.

Among animals the cases are not numerous, because no
systematic attempt has been made to select varieties for this
special quality. It has, however, been observed that, though
no European dogs thrive well in India, the Newfoundland dog,
originating from a severe climate, can hardly be kept alive.
A better case, perhaps, is furnished by merino sheep, which,
when imported directly from England, do not thrive, while those
which have been bred in the intermediate climate of the Cape
of Good Hope do much better. When geese were first intro-
duced into Bogota, they laid few eggs at long intervals, and
few of the young survived. By degrees, however, the fecundity
improved, and in about twenty years became equal to what
it is in Europe. According to Garcilaso, when fowls were
first introduced into Peru they were not fertile, whereas now
they are as much so as in Europe.

Plants furnish much more important evidence. Our
nurserymen distinguish in their catalogues varieties of fruit-
trees which are more or less hardy, and this is especially the
case in America, where certain varieties only will stand the
severe climate of Canada. There is one variety of pear, the
Forelle, which both in England and France withstood frosts

1 This account of domestic pigeons is greatly condensed from Mr.
Darwin's work already referred to.

iv VARIATION UNDER DOMESTICATION 95

that killed the flowers and buds of all other kinds of pears.
Wheat, which is grown over so large a portion of the world, has
become adapted to special climates. Wheat imported from
India and sown in good wheat soil in England produced the
most meagre ears ; while wheat taken from France to the
West Indian Islands produced either wholly barren spikes or
spikes furnished with two or three miserable seeds, while
West Indian seed by its side yielded an enormous harvest. The
orange was very tender when first introduced into Italy, and
continued so as long as it was propagated by grafts, but
when trees were raised from seed many of these were found
to be hardier, and the orange is now perfectly acclimatised in
Italy. Sweet-peas (Lathyrus odoratus) imported from England
to the Calcutta Botanic Gardens produced few blossoms and
no seed ; those from France flowered a little better, but still
produced -no seed, but plants raised from seed brought from
Darjeeling in the Himalayas, but originally derived from
England, flower and seed profusely in Calcutta.1

An observation by Mr. Darwin himself is perhaps even
more instructive. He says: "On 24th May 1864 there
was a severe frost in Kent, and two rows of scarlet runners
(Phaseolus multiflorus) in my garden, containing 390 plants of
the same age and equally exposed, were all blackened and
killed except about a dozen plants. In an adjoining row of
Fulmer's dwarf bean (Phaseolus vulgaris) one single plant
escaped. A still more severe frost occurred four days after-
wards, and of the dozen plants which had previously escaped
only three survived ; these were not taller or more vigorous
than the other young plants, but they escaped completely,
with not even the tips of their leaves browned. It was im-
possible to behold these three plants, with their blackened,
withered, and dead brethren all around them, and not see at
a glance that they differed widely in their constitutional power
of resisting frost."

The preceding sketch of the variation that occurs among
domestic animals and cultivated plants shows how wide it is
in range and how great in amount ; and we have good reason
to believe that similar variation extends to all organised beings.
In the class of fishes, for example, we have one kind which has
1 Animals and Plants under Domestication, vol. ii. pp. 307-311.

DABWINISM

been long domesticated in the East, the gold and silver carps ;
and these present great variation, not only of colour but in the
form and structure of the fins and other external organs. In
like manner, the only domesticated insects, hive bees and silk-
worm moths, present numbers of remarkable varieties which
have been produced by the selection of chance variations just
as in the case of plants and the higher animals.

Circumstances favourable to Selection by Man.

It may be supposed, that the systematic selection which
has been employed for the purpose of improving the races
of animals or plants useful to man is of comparatively recent
origin, though some of the different races are known to have
been in existence in very early times. But Mr. Darwin has
pointed out, that unconscious selection must have begun to
produce an effect as soon as plants were cultivated or animals
domesticated by man. It would have been very soon observed
that animals and plants produced their like, that seed of early
wheat produced early wheat, that the offspring of very swift
dogs were also swift, and as every one would try to have a
good rather than a bad sort this would necessarily lead to the
slow but steady improvement of all useful plants and animals
subject to man's care. Soon there would arise distinct breeds,
owing to the varying uses to which the animals and plants
were put. Dogs would be wanted chiefly to hunt one kind
of game in one part of the country and another kind else-
where ; for one purpose scent would be more important, for
another swiftness, for another strength and courage, for yet
another watchfulness and intelligence, and this Avould soon
lead to the formation of very distinct races. In the case of
vegetables and fruits, different varieties would be found to
succeed best in certain soils and climates ; some might be
preferred on account of the quantity of food they produced,
others for their sweetness and tenderness, while others might
be more useful on account of their ripening at a particular
season, and thus again distinct varieties would be established.
An instance of unconscious selection leading to distinct results
in modern times is afforded by two flocks of Leicester sheep
which both originated from the same stock, and were then bred
pure for upwards of fifty years by two gentlemen, Mr. Buckley

iv VARIATION UNDER DOMESTICATION 97

and Mr. Burgess. Mr. Youatt, one of the greatest authorities
on breeding domestic animals, says: "There is not a suspicion
existing in the mind of any one at all acquainted with the
subject that the owner of either of them has deviated in any
one instance from the pure blood of Mr. Bakewell's original
flock, and yet the difference between the sheep possessed by
these two gentlemen is so great that they have the appearance
of being quite different varieties." In this case there was no
desire to deviate from the original breed, and the difference
must have arisen from some slight difference of taste or judg-
ment in selecting, each year, the parents for the next year's
stock, combined perhaps with some direct effect of the slight
differences of climate and soil on the two farms.

Most of our domesticated animals and cultivated plants
have come to us from the earliest seats of civilisation in
Western Asia or Egypt, and have therefore been the subjects
of human care and selection for some thousands of years, the
result being that, in many cases, we do not know the wild
stock from which they originally sprang. The horse, the
camel, and the common bull and cow are nowhere found in a
wild state, and they have all been domesticated from remote
antiquity. The original of the domestic fowl is still wild in
India and the Malay Islands, and it was domesticated in India
and China before 1400 B.C. It was introduced into Europe
about 600 B.C. Several distinct breeds were known to tne
Bomans about the commencement of the Christian era, and
they have since spread all over the civilised world and been
subjected to a vast amount of conscious and unconscious
selection, to many varieties of climate and to differences of
food ; the result being seen in the wonderful diversity of breeds
which differ quite as remarkably as do the different races of
pigeons already described.

In the vegetable kingdom, most of the cereals — wheat,
barley, etc. — are unknown as truly wild plants ; and the same
is the case Avith many vegetables, for De Candolle states that
out of 157 useful cultivated plants thirty-two are quite un-
known in a wild state, and that forty more are of doubtful
origin. It is not improbable that most of these do exist
wild, but they have been so profoundly changed by thousands
of years of cultivation as to be quite unrecognisable. The

H

98 DARWINISM

peach is unknown in a wild state, unless it is derived from
the common almond, on which point there is much difference
of opinion among botanists and horticulturists.

The immense antiquity of most of our cultivated plants
sufficiently explains the apparent absence of such useful
productions in Australia and the Cape of Good Hope, not-
withstanding that they both possess an exceedingly rich and
varied flora. These countries having been, until a com-
paratively recent period, inhabited only by uncivilised men,
neither cultivation nor selection has been carried on for a
sufficiently long time. In North America, however, where
there was evidently a very ancient if low form of civilisation,
as indicated by the remarkable mounds, earthworks, and
other prehistoric remains, maize was cultivated, though it
was probably derived from Peru ; and the ancient civilisation
of that country and of Mexico has given rise to no fewer than
thirty-three useful cultivated plants.

Conditions favourable to the production of Variations.

In order that plants and animals may be improved and
modified to any considerable extent, it is of course essential
that suitable variations should occur with tolerable frequency.
There seem to be three conditions which are especially favour-
able to the production of variations: (1) That the particular
species or variety should be kept in very large numbers ; (2)
that it should be spread over a wide area and thus subjected
to a considerable diversity of physical conditions ; and (3)
that it should be occasionally crossed with some distinct but
closely allied race. The first of these conditions is perhaps
the most important, the chance of variations of any partic-
ular kind being increased in proportion to the quantity of
the original stock and of its annual offspring. It has been re-
marked that only those breeders who keep large flocks can
effect much improvement ; and it is for the same reason that
pigeons and fowls, which can be so easily and rapidly increased,
and which have been kept in such large numbers by so great
a number of persons, have produced such strange and numer-
ous varieties. In like manner, nurserymen who grow fruit and
flowers in large quantities have a great advantage over private
amateurs in the production of new varieties.

iv VARIATION UNDER DOMESTICATION 99

Although I believe, for reasons which will be given further
on, that some amount of variability is a constant and
necessary property of all organisms, yet there appears to be
good evidence to show that changed conditions of life tend to
increase it, both by a direct action on the organisation and
by indirectly affecting the reproductive system. Hence the
extension of civilisation, by favouring domestication under
altered conditions, facilitates the process of modification. Yet
this change does not seem to be an essential condition, for
nowhere has the production of extreme varieties of plants and
flowers been carried farther than in Japan, where careful
selection continued for many generations must have been the
chief factor. The effect of occasional crosses often results in
a great amount of variation, but it also leads to instability of
character, and is therefore very little employed in the pro-
duction of fixed and well-marked races. For this purpose, in
fact, it has to be carefully avoided, as it is only by isolation and
pure breeding that any specially desired qualities can be in-
creased by selection. It is for this reason that among savage
peoples, whose animals run half wild, little improvement takes
place ; and the difficulty of isolation also explains why distinct
and pure breeds of cats are so rarely met with. The wide dis-
tribution of useful animals and plants from a very remote
epoch has, no doubt, been a powerful cause of modification,
because the particular breed first introduced into each country
has often been kept pure for many years, and has also been
subjected to slight differences of conditions. It will also
usually have been selected for a somewhat different purpose
in each locality, and thus very distinct races would soon
originate.

The important physiological effects of crossing breeds or
strains, and the part this plays in the economy of nature, will
be explained in a future chapter.

Concluding Remarks.

The examples of variation now adduced — and these might
have been almost indefinitely increased — will suffice to show
that there is hardly an organ or a quality in plants or animals
which has not been observed to vary ; and further, that when-
ever any of these variations have been useful to man he has

100 DARWINISM

been able to increase them to a marvellous extent by the
simple process of always preserving the best varieties to breed
from. Along with these larger variations others of smaller
amount occasionally appear, sometimes in external, sometimes
in internal characters, the very bones of the skeleton often
changing slightly in form, size, or number; but as these
secondary characters have been of no use to man, and have
not been specially selected by him, they have, usually, not
been developed to any great amount except when they have
been closely dependent on those external characters which he
has largely modified.

As man has considered only utility to himself, or the
satisfaction of his love of beauty, of novelty, or merely of
something strange or amusing, the variations he has thus pro-
duced have something of the character of monstrosities. Not
only are they often of no use to the animals or plants them-
selves, but they are not unfrequently injurious to them. In
the Tumbler pigeons, for instance, the habit of tumbling is
sometimes so excessive as to injure or kill the bird ; and many
of our highly-bred animals have such delicate constitutions
that they are very liable to disease, while their extreme
peculiarities of form or structure would often render them
quite unfit to live in a wild state. In plants, many of our
double flowers, and some fruits, have lost the power of pro-
ducing seed, and the race can thus be continued only by means
of cuttings or grafts. This peculiar character of domestic
productions distinguishes them broadly from wild species and
varieties, which, as will be seen by and by, are necessarily
adapted in every part of their organisation to the conditions
under which they have to live. Their importance for our
present inquiry depends on their demonstrating the occurrence
of incessant slight variations in all parts of an organism, with
the transmission to the offspring of the special characteristics
of the parents ; and also, that all such slight variations are
capable of being accumulated by selection till they present
very large and important divergencies from the ancestral
stock.

We thus see, that the evidence as to variation afforded
by animals and plants under domestication strikingly accords
with that which we have proved to exist in a state of nature.

iv VARIATION UNDER DOMESTICATION 101

And it is not at all surprising that it should be so, since all
the species were in a state of nature when first domesticated
or cultivated by man, and whatever variations occur must be
due to purely natural causes. Moreover, on comparing the
variations which occur in any one generation of domesticated
animals with those which we know to occur in wild animals,
we find no evidence of greater individual variation in the
former than in the latter. The results of man's selection are
more striking to us because we have always considered the
varieties of each domestic animal to be essentially identical,
while those which we observe in a wild state are held to be
essentially diverse. The greyhound and the spaniel seem
wonderful, as varieties of one animal produced by man's
selection ; while we think little of the diversities of the fox
and the wolf, or the horse and the zebra, because we have
been accustomed to look upon them as radically distinct
animals, not as the results of nature's selection of the
varieties of a common ancestor.

CHAPTEE V

NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE

FITTEST

Effect of struggle for existence under unchanged conditions — The effect
under change of conditions — Divergence of character — -In insects — In
birds — In mammalia — Divergence leads to a maximum of life in each
area — Closely allied species inhabit distinct areas — Adaptation to
conditions at various periods of life — The continued existence of low
forms of life — Extinction of low types among the higher animals —
Circumstances favourable to the origin of new species — Probable origin
of the dippers — The importance of isolation — On the advance of organi-
sation by natural selection — Summary of the first five chapters.

In the preceding chapters we have accumulated a body of
facts and arguments which will enable us now to deal with the
very core of our subject — the formation of species by means
of natural selection. We have seen how tremendous is the
struggle for existence always going on in nature owing to the
great powers of increase of all organisms ; we have ascertained
the fact of variability extending to every part and organ, each
of which varies simultaneously and for the most part independ-
ently ; and we have seen that this variability is both large in
its amount in proportion to the size of each part, and usually
affects a considerable proportion of the individuals in the large
and dominant species. And, lastly, we have seen how similar
variations, occurring in cultivated plants and domestic animals,
are capable of being perpetuated and accumulated by artificial
selection, till they have resulted in all the wonderful varieties
of our fruits, flowers, and vegetables, our domestic animals and
household pets, many of which differ from each other far more
in external characters, habits, and instincts than do species in

chap, v NATURAL SELECTION 103

a state of nature. We have now to inquire whether there is
any analogous process in nature, by which wild animals and
plants can be permanently modified and new races or new
species produced.

Effect of Struggle for Existence under Unchanged Conditions.

Let us first consider what will be the effect of the struggle
for existence upon the animals and plants which we see around
us, under conditions which do not perceptibly vary from year
to year or from century to century. We have seen that every
species is exposed to numerous and varied dangers throughout
its entire existence, and that it is only by means of the exact
adaptation of its organisation — including its instincts and habits
— to its surroundings that it is enabled to live till it produces
offspring which may take its place when it ceases to exist.
We have seen also that, of the whole annual increase only a
very small fraction survives ; and though the survival in indi-
vidual cases may sometimes be due rather to accident than
to any real superiority, yet we cannot doubt that, in the long
run, those survive which are best fitted by their perfect organisa-
tion to escape the dangers that surround them. This " survival
of the fittest" is what Darwin termed "natural selection,"
because it leads to the same results in nature as are produced
by man's selection among domestic animals and cultivated
plants. Its primary effect will, clearly, be to keep each species
in the most perfect health and vigour, with every part of its
organisation in full harmony with the conditions of its existence.
It prevents any possible deterioration in the organic world, and
produces that appearance of exuberant life and enjoyment, of
health and beauty, that affords us so much pleasure, and which
might lead a superficial observer to suppose that peace and
quietude reigned throughout nature.

The Effect under changed Conditions.

But the very same process which, so long as conditions re-
main substantially the same, secures the continuance of each
species of animal or plant in its full perfection, will usually,
under changed conditions, bring about whatever change of
structure or habits may be necessitated by them. The changed
conditions to which we refer are such as we know have occurred

104 DARWINISM

throughout all geological time and in every part of the world.
Land and water have been continually shifting their positions ;
some regions are undergoing subsidence with diminution of
area, others elevation with extension of area ; dry land has
been converted into marshes, while marshes have been drained
or have even been elevated into plateaux. Climate too has
changed again and again, either through the elevation of
mountains in high latitudes leading to the accumulation of
snow and ice, or by a change in the direction of Avinds and
ocean currents produced by the subsidence or elevation of lands
which connected continents and divided oceans. Again, along
with all these changes have come not less important changes
in the distribution of species. Vegetation has been greatly
modified by changes of climate and of altitude ; while every
union of lands before separated has led to extensive migrations
of animals into new countries, disturbing the balance that
before existed among its forms of life, leading to the extermina-
tion of some species and the increase of others.

When such physical changes as these have taken place, it is
evident that many species must either become modified or
cease to exist. When the vegetation has changed in character
the herbivorous animals must become able to live on new and
perhaps less nutritious food ; while the change from a damp
to a dry climate may necessitate migration at certain periods
to escape destruction by drought. This will expose the species
to new dangers, and require special modifications of structure
to meet them. Greater swiftness, increased cunning, nocturnal
habits, change of colour, or the power of climbing trees and
living for a time on their foliage or fruit, may be the means
adopted by different species to bring themselves into harmony
with the new conditions ; and by the continued survival of
those individuals, only, which varied sufficiently in the right
direction, the necessary modifications of structure or of func-
tion would be brought about, just as surely as man has been
able to breed the greyhound to hunt by sight and the fox-
hound by scent, or has produced from the same wild plant
such distinct forms as the cauliflower and the brussels sprouts.

We will now consider the special characteristics of the
changes in species that are likely to be effected, and how far
they agree with what we observe in nature.

v NATURAL SELECTION 105

Divergence of Character.

In species which have a wide range the struggle for exist-
ence will often cause some individuals or groups of individuals
to adopt new habits in order to seize upon vacant places in
nature where the struggle is less severe. Some, living among
extensive marshes, may adopt a more aquatic mode of life ;
others, living where forests abound, may become more arboreal.
In either case we cannot doubt that the changes of structure
needed to adapt them to their new habits would soon be
brought about, because we know that variations in all the
external organs and all their separate parts are very abundant
and are also considerable in amount. That such divergence of
character has actually occurred we have some direct evidence.
Mr. Darwin informs us that in the Catskill Mountains in the
United States there are two varieties of wolves, one with a
light greyhound-like form which pursues deer, the other more
bulky with shorter legs, which more frequently attacks sheep.1
Another good example is that of the insects in the island of
Madeira, many of which have either lost their wings or have
had them so much reduced as to be useless for flight, while the
very same species on the continent of Europe possess fully
developed wings. In other cases the wingless Madeira species
are distinct from, but closely allied to, winged species of Europe.
The explanation of this change is, that Madeira, like many
oceanic islands in the temperate zone, is much exposed to
sudden gales of wind, and as most of the fertile land is on the
coast, insects which flew much would be very liable to be
blown out to sea and lost. Year after year, therefore, those
individuals which had shorter wings, or which used them least,
were preserved ; and thus, in time, terrestrial, wingless, or im-
perfectly winged races or species have been produced. That
this is the true explanation of this singular fact is proved by
much corroborative evidence. There are some few flower-
frequenting insects in Madeira to whom wings are essential,
and in these the wings are somewhat larger than in the same
species on the mainland. We thus see that there is no general
tendency to the abortion of wings in Madeira, but that it is
simply a case of adaptation to new conditions. Those insects

1 Origin of Species, p. 71.

106 DARWINISM

to whom wings were not absolutely essential escaped a serious
clanger by not using them, and the wings therefore became
reduced or were completely lost. But when they were essential
they were enlarged and strengthened, so that the insect could
battle against the winds and save itself from destruction at
sea. Many flying insects, not varying fast enough, would be
destroyed before they could establish themselves, and thus we
may explain the total absence from Madeira of several whole
families of winged insects which must have had many oppor-
tunities of reaching the islands. Such are the large groups of
the tiger-beetles (Cicindelidse), the chafers (Melolonthidas), the
click-beetles (Elateridse), and many others.

But the most curious and striking confirmation of this
portion of Mr. Darwin's theory is afforded by the case of
Kerguelen Island. This island was visited by the Transit of
Venus expedition. It is one of the stormiest places on the
globe, being subject to almost perpetual gales, while, there
being no wood, it is almost entirely without shelter. The
Rev. A. E. Eaton, an experienced entomologist, was naturalist
to the expedition, and he assiduously collected the few insects
that were to be found. All were incapable of flight, and most
of them entirely without wings. They included a moth,
several flies, and numerous beetles. As these insects could
hardly have reached the islands in a wingless state, even if
there were any other known land inhabited by them — which
there is not — we must assume that,, like the Madeiran insects,
they were originally winged, and lost their power of flight
because its possession was injurious to them.

It is no doubt clue to the same cause that some butterflies
on small and exposed islands have their wings reduced in size,
as is strikingly the case with the small tortoise-shell butterfly
(Vanessa urticse) inhabiting the Isle of Man, which is only
about half the size of the same species in England or Ireland ;
and Mr. Wollaston notes that Vanessa callirhoe — a closely allied
South European form of our red-aclmiral butterfly— is perma-
nently smaller in the small and bare island of Porto Santo
than in the larger and more wooded adjacent island of Madeira.

A very good example of comparatively recent divergence
of character, in accordance with new conditions of life, is
afforded by our red grouse. This bird, the Lagopus scoticus of

V NATURAL SELECTION 107

naturalists, is entirely confined to the British Isles. It is,
however, very closely allied to the willow grouse (Lagopus
albus), a bird which ranges all over Europe, Northern Asia,
and North America, but which, unlike our species, changes to
white in winter. No difference in form or structure can be
detected between the two birds, but as they differ so decidedly
in colour — our species being usually rather darker in winter
than in summer, while there are also slight differences in the
call-note and in habits, — the two species are generally con-
sidered to be distinct. The differences, however, are so
clearly adaptations to changed conditions that we can hardly
doubt that, during the early part of the glacial period, when
our islands were united to the continent, our grouse was
identical with that of the rest of Europe. But when the cold
passed away and our islands became permanently separated
from the mainland, with a mild and equable climate and very
little snow in winter, the change to white at that season
became hurtful, rendering the birds more conspicuous instead
of serving as a means of concealment. The colour was, there-
fore, gradually changed by the process of variation and natural
selection ; and as the birds obtained ample shelter among the
heather which clothes so many of our moorlands, it became
useful for them to assimilate with its brown and dusky stems
and withered flowers rather than with the snow of the higher
mountains. An interesting confirmation of this change having
really occurred is afforded by the occasional occurrence in
Scotland of birds with a considerable amount of white in the
winter plumage. This is considered to be a case of reversion
to the ancestral type, just as the slaty colours and banded
wings of the wild rock-pigeon sometimes reappear in our fancy
breeds of domestic pigeons.1

The principle of " divergence of character " pervades all
nature from the lowest groups to the highest, as may be
well seen in the class of birds. Among our native species we
see it well marked in the different species of titmice, pipits,
and chats. The great titmouse (Parus major) by its larger
size and stronger bill is adapted to feed on larger insects, and
is even said sometimes to kill small and weak birds. The
smaller and weaker coal titmouse (Parus ater) has adopted a

1 Yarrell's British Birds, fourth edition, vol. iii. p. 77.

108 DARWINISM

more vegetarian diet, eating seeds as well as insects, and
feeding on the ground as well as among trees. The delicate
little blue titmouse (Parus cceruleus), with its very small bill,
feeds on the minutest insects and grubs which it extracts
from crevices of , bark and from the buds of fruit-trees. The
marsh titmouse, again (Parus palustris), has received its name
from the low and marshy localities it frequents ; while the
crested titmouse (Parus cristatus) is a northern bird frequenting
especially pine forests, on the seeds of which trees it partially
feeds. Then, again, our three common pipits — the tree-pipit
( Anthus arboreus), the meadow-pipit (Anthus pratensis), and the
rock-pipit or sea-lark (Anthus obscurus) have each occupied a
distinct place in nature to which they have become specially
adapted, as indicated by the different form and size of the
hind toe and claw in each species. So, the stone-chat (Saxicola
rubicola), the whin-chat (S. rubetra), and the wheat-ear (S.
cenanthe) are more or less divergent forms of one type, with
modifications in the shape of the wing, feet, and bill adapting
them to slightly different modes of life. The whin-chat is the
smallest, and frequents furzy commons, fields, and loAvlands,
feeding on worms, insects, small molluscs, and berries ; the
stone-chat is next in size, and is especially active and lively,
frequenting heaths and uplands, and is a permanent resident
with us, the two other species being migrants ; while the
larger and more conspicuous wheat-ear, besides feeding on
grubs, beetles, etc., is able to capture flying insects on the
wing, something after the manner of true flycatchers.

These examples sufficiently indicate how divergence of
character has acted, and has led to the adaptation of numerous
allied species, each to a more or less special mode of life, with
the variety of food, of habits, and of enemies which must
necessarily accompany such diversity. And when Ave extend
our inquiries to higher groups we find the same indications of
divergence and special adaptation, often to a still more marked
extent. Thus we have the larger falcons, which prey upon
birds, while some of the smaller species, like the hobby
(Falco subbuteo), live largely on insects. The true falcons
capture their prey in the air, while the hawks usually seize it
on or near the ground, feeding on hares, rabbits, squirrels,
grouse, pigeons, and poultry. Kites and buzzards, on the

V NATURAL SELECTION 109

other hand, seize their prey upon the ground, and the former
feed largely on reptiles and offal as well as on birds and
quadrupeds. Others have adopted fish as their chief food,
and the osprey snatches its prey from the water with as much
facility as a gull or a petrel; while the South American
caracaras (Polyborus) have adopted the habits of vultures and
live altogether on carrion. In every great group there is the
same divergence of habits. There are ground-pigeons, rock-
pigeons, and wood-pigeons, — seed-eating pigeons and fruit-
eating pigeons ; there are carrion-eating, insect-eating, and
fruit-eating crows. Even kingfishers are, some aquatic, some
terrestrial in their habits ; some live on fish, some on insects,
some on reptiles. Lastly, among the primary divisions of birds
we find a purely terrestrial group — the Batitse, including the
ostriches, cassowaries, etc. ; other great groups, including the
ducks, cormorants, gulls, penguins, etc., are aquatic ; while the
bulk of the Passerine birds are aerial and arboreal. The
same general facts can be detected in all other classes of
animals. In the mammalia, for example, we have in the common
rat a fish-eater and flesh-eater as well as a grain-eater, which
has no doubt helped to give it the power of spreading over
the world and driving away the native rats of other countries.
Throughout the Eodent tribe we find everywhere aquatic,
terrestrial, and arboreal forms. In the weasel and cat tribes
some live more in trees, others on the ground ; squirrels have
diverged into terrestrial, arboreal, and flying species ; and
finally, in the bats we have a truly aerial, and in the whales
a truly aquatic order of mammals. We thus see that,
beginning with different varieties of the same species, we
have allied species, genera, families, and orders, with similarly
divergent habits, and adaptations to different modes of life,
indicating some general principle in nature which has been
operative in the development of the organic world. But in
order to be thus operative it must be a generally useful
principle, and Mr. Darwin has very clearly shown us in what
this utility consists.

Divergence leads to a Maximum of Organic Forms in each Area.

Divergence of character has a double purpose and use. In
the first place it enables a species which is being overcome

110 DARWINISM chap.

by rivals, or is in process of extinction by enemies, to save
itself by adopting new habits or by occupying vacant places
in nature. This is the immediate and obvious effect of all
the numerous examples of divergence of character which Ave
have pointed out. But there is another and less obvious
result, which is, that the greater the diversity in the organisms
inhabiting a country or district the greater will be the total
amount of life that can be supported there. Hence the
continued action of the struggle for existence will tend to
bring about more and more diversity in each area, which may
be shown to be the case by several kinds of evidence. As an
example, a piece of turf, three feet by four in size, was found
by Mr. Darwin to contain twenty species of plants, and these
twenty species belonged to eighteen genera and to eight
orders, showing how greatly they differed from each other.
Farmers find that a greater quantity of hay is obtained from
ground sown with a variety of genera of grasses, clover,
etc., than from similar land sown with one or two species
only ; and the same principle applies to rotation of crops,
plants differing very widely from each other giving the
best results. So, in small and uniform islands, and in
small ponds of fresh water, the plants and insects, though
few in number, are found to be wonderfully varied in
character.

The same principle is seen in the naturalisation of plants
and animals by man's agency in distant lands, for the species
that thrive best and establish themselves permanently are
not only very varied among themselves but differ greatly from
the native inhabitants. Thus, in the Northern United States
there are, according to Dr. Asa Gray, 260 naturalised flower-
ing plants which belong to no less than 162 genera ; and of
these, 1 00 genera are. not natives of the United States. So, in
Australia, the rabbit, though totally unlike any native animal,
has increased so much that it probably outnumbers in in-
dividuals all the native mammals of the country; and in
New Zealand the rabbit and the pig have equally multiplied.
Darwin remarks that this "advantage of diversification of
structure in the inhabitants of the same region is, in fact, the
same as that of the physiological division of labour in the
organs of the same body. No physiologist doubts that a

V NATURAL SELECTION 111

stomach adapted to digest vegetable matter alone, or flesh
alone, draws more nutriment from these substances. So, in
the general economy of any land, the more widely and
perfectly the animals and plants are diversified for different
habits of life, so will a greater number of individuals be
capable of there supporting themselves."1

The most closely allied Species inhabit distinct Areas.

One of the curious results of the general action of this
principle in nature is, that the most closely allied species —
those whose differences though often real and important are
hardly perceptible to any one but a naturalist — are usually
not found in the same but in widely separated countries.
Thus, the nearest allies to our European golden plover are
found in North America and East Asia ; the nearest ally
of our European jay is found in Japan, although there are
several other species of jays in Western Asia and North
Africa ; and though we have several species of titmice in
England they are not very closely allied to each other.
The form most akin to our blue tit is the azure tit of
Central Asia (Parus azureus) ; the Parus ledouci of Algeria
is very near our coal tit, and the Parus lugubris of South-
Eastern Europe and Asia Minor is nearest to our marsh tit.
So, our four species of wild pigeons — the ring-dove, stock-
dove, rock-pigeon, and turtle-dove — are not closely allied to
each other, but each of them belongs, according to some
ornithologists, to a separate genus or subgenus, and has its
nearest relatives in distant parts of Asia and Africa. In
mammalia the same thing occurs. Each mountain region of
Europe and Asia has usually its own species of wild sheep
and goat, and sometimes of antelope and deer ; so that in
each region there is found the greatest diversity in this
class of animals, while the closest allies inhabit quite distinct
and often distant areas. In plants we find the same
phenomenon prevalent. Distinct species of columbine are
found in Central Europe (Aguilegia vulgaris), in Eastern
Europe, and Siberia (A. glandulosa), in the Alps (A. Alpina),
in the Pyrenees (A. pyrenaiea), in the Greek mountains (A.
ottonis), and in Corsica (A. Bernardi), but rarely are two
1 Origin of Species, p. 89.

112 DARWINISM chap.

species found in the same area. So, each part of the
world has its own peculiar forms of pines, firs, and cedars,
but the closely allied species or varieties are in almost
every case inhabitants of distinct areas. Examples are the
deodar of the Himalayas, the cedar of Lebanon, and that of
North Africa, all very closely allied but confined to distinct
areas ; and the numerous closely allied species of true pine
(genus Pinus), which almost always inhabit different countries
or occupy different stations. We will now consider some
other modes in which natural selection will act, to adapt
organisms to changed conditions.

Adaptation to Conditions at Various Periods of Life.

It is found, that, in domestic animals and cultivated plants,
variations occurring at any one period of life reappear in the
offspring at the same period, and can be perpetuated and
increased by selection without modifying other parts of the
organisation. Thus, variations in the caterpillar or the cocoon
of the silkworm, in the eggs of poultry, and in the seeds
or young shoots of many culinary vegetables, have been
accumulated till those parts have become greatly modified and,
for man's purposes, improved. Owing to this fact it is easy
for organisms to become so modified as to avoid dangers that
occur at any one period of life. Thus it is that so many
seeds have become adapted to various modes of dissemination
or protection. Some are winged, or have down or hairs
attached to them, so as to enable them to be carried long-
distances in the air ; others have curious hooks and prickles,
which cause them to be attached firmly to the fur of mammals
or the feathers of birds ; while others are buried within sweet
or juicy and brightly coloured fruits, which are seen and
devoured by birds, the hard smooth seeds passing through
their bodies in a fit state for germination. In the struggle
for existence it must benefit a plant to have increased means
of dispersing its seeds, and of thus having young plants pro-
duced in a greater variety of soils, aspects, and surroundings,
with a greater chance of some of them escaping their numerous
enemies and arriving at maturity. The various differences
referred to would, therefore, be brought about by variation and
survival of the fittest, just as surely as the length and quality

v NATURAL SELECTION 113

of cotton on the seed of the cotton-plant have been increased
by man's selection.

The larvae of insects have thus been wonderfully modified
in order to escape the numerous enemies to whose attacks
they are exposed at this period of their existence. Their
colours and markings have become marvellously adapted to
conceal them among the foliage of the plant they live upon,
and this colour often changes completely after the last moult,
when the creature has to descend to the ground for its change
to the pupa state, during which period a brown instead of a
green colour is protective. Others have acquired curious
attitudes and large ocelli, which cause them to resemble the
head of some reptile, or they have curious horns or coloured
ejectile processes which frighten away enemies ; while a great
number have acquired secretions which render them offensive
to the taste of their enemies, and these are always adorned
with very conspicuous markings or brilliant colours, which
serve as a sign of inedibility and prevent their being needlessly
attacked. This, however, is a portion of the very large sub-
ject of organic colour and marking, which will be fully dis-
cussed and illustrated in a separate chapter.

In this way every possible modification of an animal or
plant, whether in colour, form, structure, or habits, which
would be serviceable to it or to its progeny at any period of
its existence, may be readily brought about. There are some
curious organs which are used only once in a creature's life,
but which are yet essential to its existence, and thus have
very much the appearance of design by an intelligent designer.
Such are, the great jaws possessed by some insects, used ex-
clusively for opening the cocoon, and the hard tip to the beak
of unhatched birds used for breaking the eggshell. The
increase in thickness or hardness of the cocoons or the eggs
being useful for protection against enemies or to avoid
accidents, it is probable that the change has been very
gradual, because it would be constantly checked by the
necessity for a corresponding change in the young insects or
birds enabling them to overcome the additional obstacle of a
tougher cocoon or a harder eggshell. As we have seen,
however, that every part of the organism appears to be
varying independently, at the same time, though to different

I

114 DARWINISM

amounts, there seems no reason to believe that the necessity
for two or more coincident variations would prevent the
required change from taking place.

The Continued Existence of Low Forms of Life.

Since species are continually undergoing modifications
giving them some superiority over other species or enabling
them to occupy fresh places in nature, it may be asked — Why
do any low forms continue to exist 1 Why have they not long-
since been improved and developed into higher forms ? The
answer, probably, is, that these low forms occupy places in
nature which cannot be filled by higher forms, and that they
have few or no competitors ; they therefore continue to
exist. Thus, earthworms are adapted to their mode of life
better than they would be if more highly organised. So, in
the ocean, the minute foraminifera and infusoria, and the
larger sponges and corals, occupy places which more highly
developed creatures could not fill. They form, as it were, the
base of the great structure of animal life, on which the next
higher forms rest ; and though in the course of ages they
may undergo some changes, and diversification of form
and structure, in accordance with changed conditions, their
essential nature has probably remained the same from the
very dawn of life on the earth. The low aquatic diatomacese
and conferva?, together with the lowest fungi and lichens,
occupy a similar position in the vegetable kingdom, filling
places in nature which would be left vacant if only highly
organised plants existed. There is, therefore, no motive
power to destroy or seriously to modify them ; and they have
thus probably persisted, under slightly varying forms, through
all geological time.

Extinction of Lower Types among the Higher Animals.

So soon, however, as we approach the higher and more
fully developed groups, we see indications of the often re-
peated extinction of lower by higher forms. This is shown
by the great gaps that separate the mammalia, birds, reptiles,
and fishes from each other ; while the lowest forms of each are
always few in number and confined to limited areas. Such

v NATURAL SELECTION 115

are the lowest mammals — the echidna and ornithorhynchus of
Australia; the lowest birds — the apteryx of New Zealand
and the cassowaries of the New Guinea region ; while the
lowest fish — the amphioxus or lancelet, is completely isolated,
and has apparently survived only by its habit of burrow-
ing in the sand. The great distinctness of the carnivora,
ruminants, rodents, whales, bats, and other orders of
mammalia ; of the accipitres, pigeons, and parrots, among
birds ; and of the beetles, bees, flies, and moths, among insects,
all indicate an enormous amount of extinction among the
comparatively low forms by which, on any theory of evolution,
these higher and more specialised groups must have been
preceded.

Circumstances favourable to the Origin of New Species by
Natural Selection.

We have already seen that, when there is no change in
the physical or organic conditions of a country, the effect of
natural selection is to keep all the species inhabiting it in a
state of perfect health and full development, and to preserve
the balance that already exists between , the different groups
of organisms. But, whenever the physical or organic condi-
tions change, to however small an extent, some correspond-
ing change will be produced in the flora and fauna, since,
considering the severe struggle for existence and the complex
relations of the various organisms, it is hardly possible that
the change should not be beneficial to some species and
hurtful to others. The most common effect, therefore, will
be that some species will increase and others will diminish ;
and in cases where a species was already small in numbers a
further diminution might lead to extinction. This would
afford room for the increase of other species, and thus a
considerable readjustment of the proportions of the several
species might take place. When, however, the change was of
a more important character, directly affecting the existence of
many species so as to render it difficult for them to maintain
themselves without some considerable change in structure or
habits, that change would, in some cases, be brought about by
variation and natural selection, and thus new varieties or new
species might be formed. We have to consider, then, which

116 DARWINISM

are the species that would be most likely to be so modified,
while others, not becoming modified, would succumb to the
changed conditions and become extinct.

The most important condition of all is, undoubtedly, that
variations should occur of sufficient amount, of a sufficiently
diverse character, and in a large number of individuals, so as
to afford ample materials for natural selection to act upon ;
and this, we have seen, does occur in most, if not in all, large,
wide-ranging, and dominant species. From some of these,
therefore, the new species adapted to the changed conditions
would usually be derived ; and this would especially be the
case when the change of conditions was rather rapid, and when
a correspondingly rapid modification could alone save some
species from extinction. But when the change was very
gradual, then even less abundant and less widely distributed
species might become modified into new forms, more especially
if the extinction of many of the rarer species left vacant
places in the economy of nature.

Probable Origin of the Dippers.

An excellent example of how a limited group of species
has been able to maintain itself by adaptation to one of
these " vacant places " in nature, is afforded by the curious
little birds called dippers or water- ouzels, forming the genus
Cinclus and the family Cinclid?e of naturalists. These birds
are something like small thrushes, with very short wings and
tail, and very dense plumage. They frequent, exclusively,
mountain torrents in the northern hemisphere, and obtain
their food entirely in the water, consisting, as it does, of water-
beetles, caddis -worms and other insect -larvae, as well as
numerous small fresh- water shells. These birds, although not
far removed in structure from thrushes and wrens, have the
extraordinary power of flying under water ; for such, ac-
cording to the best observers, is their process of diving in
search of their prey, their dense and somewhat fibrous
plumage retaining so much air that the water is prevented
from touching their bodies or even from wetting their feathers
to any great extent. Their powerful feet and long curved
claws enable them to hold on to stones at the bottom, and
thus to retain their position while picking up insects, shells,

V NATURAL SELECTION 117

etc. As they frequent chiefly the most rapid and boisterous
torrents, among rocks, waterfalls, and huge boulders, the
water is never frozen over, and they are thus able to live
during the severest winters. Only a very few species of
dipper are known, all those of the old world being so closely
allied to our British bird that some ornithologists consider
them to be merely local races of one species ; while in North
America and the northern Andes there are two other
species.

Here then we have a bird, which, in its whole structure,
shows a close affinity to the smaller typical perching birds,
but which has departed from all its allies in its habits and
mode of life, and has secured for itself a place in nature
where it has few competitors and few enemies. We may
Ayell suppose, that, at some remote period, a bird which was
perhaps the common and more generalised ancestor of most
of our thrushes, warblers, wrens, etc., had spread widely over
the great northern continent, and had given rise to numerous
varieties adapted to special conditions of life. Among these
some took to feeding on the borders of clear streams, picking
out such larvae and molluscs as they could reach in shallow
water. When food became scarce they would attempt to
pick them out of deeper and deeper water, and while doing
this in cold weather many would become frozen and starved.
But any which possessed denser and more hairy plumage
than usual, which was able to keep out the water, would
survive ; and thus a race would be formed which would depend
more and more on this kind of food. Then, following up the
frozen streams into the mountains, they would be able to live
there during the winter; and as such places afforded them much
protection from enemies and ample shelter for their nests and
young, further adaptations would occur, till the wonderful
power of diving and flying under water was acquired by a
true land-bird.

That such habits might be acquired under stress of need
is rendered highly probable by the facts stated by the well-
known American naturalist, Dr. Abbott. He says that "the
water -thrushes (Seiurus sp.) all wade in water, and often,
seeing minute mollusca on the bottom of the stream, plunge
both head and neck beneath the surface, so that often, for

118 DARWINISM

several seconds, a large part of the body is submerged. Now
these birds still have the plumage pervious to water, and so
are liable to be drenched and sodden ; but they have also the
faculty of giving these drenched feathers such a good shaking
that flight is practicable a moment after leaving the water.
Certainly the water- thrushes (Seiurus ludovicianus, S. aurica-
pillus, and S. noveboracensis) have taken many preliminary
steps to becoming as aquatic as the dipper ; and the winter-
wren, and even the Maryland yellow-throat are not far
behind." 1

Another curious example of the way in which species have
been modified to occupy new places in nature, is afforded by
the various animals which inhabit the water -vessels formed
by the leaves of many epiphytal species of Bromelia. Fritz
Miiller has described a caddis-fly larva which lives among these
leaves, and which has been modified in the pupa state in
accordance with its surroundings. The pupae of caddis-flies
inhabiting streams have fringes of hair on the tarsi to enable
them to reach the surface on leaving their cases. But in the
species inhabiting bromelia leaves there is no need for swimming,
and accordingly we find the tarsi entirely bare. In the same
plants are found curious little Entomostraca, very abundant
there but found nowhere else. These form a new genus, but
are most nearly allied to Cythere, a marine type. It is believed
that the transmission of this species from one tree to another
must be effected by the young Crustacea, which are very
minute, clinging to beetles, many of which, both terrestrial and
aquatic, also inhabit the bromelia leaves ; and as some water-
beetles are known to frequent the sea, it is perhaps by these
means that the first emigrants established themselves in this
strange new abode. Bromelise are often very abundant on trees
growing on the water's edge, and this would facilitate the tran-
sition from a marine to an arboreal habitat. Fritz Miiller has
also found, among the bromelia leaves, a small frog bearing
its eggs on its back, and having some other peculiarities of
structure. Several beautiful little aquatic plants of the genus
TJtricularia or bladder -wort also inhabit bromelia leaves ; and
these send runners out to neighbouring plants and thus spread
themselves with great rapidity.

1 Nature, vol. xxx. p. 30.

v NATURAL SELECTION 119

The Importance of Isolation.

Isolation is no doubt an important aid to natural selection,
as shown by the fact that islands so often present a number
of peculiar species ; and the same thing is seen on the two
sides of a great mountain range or on opposite coasts of a
continent. The importance of isolation is twofold. In the
first place, it leads to a body of individuals of each species being
limited in their range and thus subjected to uniform condi-
tions for long spaces of time. Both the direct action of the
environment and the natural selection of such varieties only
as are suited to the conditions, will, therefore, be able to
produce their full effect. In the second place, the process of
change will not be interfered with by intercrossing with other
individuals which are becoming adapted to somewhat different
conditions in an adjacent area. But this question of the
swamping effects of intercrossing will be considered in another
chapter.

Mr. Darwin was of opinion that, on the whole, the largeness
of the area occupied by a species was of more importance than
isolation, as a factor in the production of new species, and in
this I quite agree with him. It must, too, be remembered,
that isolation will often be produced in a continuous area
whenever a species becomes modified in accordance with varied
conditions or diverging habits. For example, a wide-ranging
species may in the northern and colder part of its area become
modified in one direction, and in the southern part in another
direction ; and though for a long time an intermediate form
may continue to exist in the intervening area, this will be
likely soon to die out, both because its numbers will be small,
and it will be more or less pressed upon in varying seasons by
the modified varieties, each better able to endure extremes of
climate. So, Avhen one portion of a terrestrial species takes to
a more arboreal or to a more aquatic mode of life, the change
of habit itself leads to the isolation of each portion. Again,
as will be more fully explained in a future chapter, any
difference of habits or of haunts usually leads to some modi-
fication of colour or marking, as a means of concealment from
enemies ; and there is reason to believe that this difference will
be intensified by natural selection as a means of identification

120 DARWINISM chap.

and recognition by members of the same variety or incipient
species. It has also been observed that each differently
coloured variety of wild animals, or of domesticated animals
which have run wild, keep together, and refuse to pair with
individuals of the other colours ; and this must of itself act to
keep the races separate as completely as physical isolation.

On the Advance of Organisation by Natural Selection.

As natural selection acts solely by the preservation of use-
ful variations, or those which are beneficial to the organism
under the conditions to which it is exposed, the result must
necessarily be that each species or group tends to become more
and more improved in relation to its conditions. Hence we
should expect that the larger groups in each class of animals
and plants — those which have persisted and have been abundant
throughout geological ages — would, almost necessarily, have
arrived at a high degree of organisation, both physical and
mental. Illustrations of this are to be seen everywhere.
Among mammalia we have the carnivora, which from Eocene
times have been becoming more and more specialised, till they
have culminated in the cat and dog tribes, which have reached
a degree of perfection both in structure and intelligence fully
equal to that of any other animals. In another line of
development, the herbivora have been specialised for living
solely on vegetable food till they have culminated in the sheep,
the cattle, the deer, and the antelopes. The horse tribe,
commencing with an early four-toed ancestor in the Eocene
age, has increased in size and in perfect adaptation of feet and
teeth to a life on open plains, and has reached its highest per-
fection in the horse, the ass, and the zebra. In birds, also, we
see an advance from the imperfect tooth-billed and reptile-
tailed birds of the secondary epoch, to the wonderfully
developed falcons, crows, and swallows of our time. So, the
ferns, lycopods, conifers, and monocotyledons of the palreozoic
and mesozoic rocks, have developed into the marvellous wealth
of forms of the higher dicotyledons that now adorn the earth.

But this remarkable advance in the higher and larger groups
does not imply any universal law of progress in organisation,
because we have at the same time numerous examples (as has
been already pointed out) of the persistence of lowly organised

V NATURAL SELECTION 121

forms, and also of absolute degradation or degeneration. Ser-
pents, for example, have been developed from some lizard-like
type which has lost its limbs ; and though this loss has enabled
them to occupy fresh places in nature and to increase and
nourish to a marvellous extent, yet it must be considered to be
a retrogression rather than an advance in organisation. The
same remark will apply to the whale tribe among mammals ;
to the blind amphibia and insects of the great caverns ; and
among plants to the numerous cases in which flowers, once
specially adapted to be fertilised by insects, have lost their
gay corollas and their special adaptations, and have become
degraded into wind-fertilised forms. Such are our plantains,
our meadow burnet, and even, as some botanists maintain, our
rushes, sedges, and grasses. The causes which have led to
this degeneration will be discussed in a future chapter ; but
the facts- are undisputed, and they show us that although
variation and the struggle for existence may lead, on the
whole, to a continued advance of organisation ; yet they also
lead in many cases to a retrogression, when such retrogression
may aid in the preservation of any form under new conditions.
They also lead to the persistence, with slight modifications, of
numerous lowly organised forms which are suited to places
which higher forms could not fully occupy, or to conditions
under which they could not exist. Such are the ocean
depths, the soil of the earth, the mud of rivers, deep caverns,
subterranean waters, etc. ; and it is in such places as these, as
well as in some oceanic islands which competing higher forms
have not been able to reach, that we find many curious relics
of an earlier world, which, in the free air and sunlight and in
the great continents, have long since been driven out or exter-
minated by higher types.

Summary of the first Five Chapters.

We have now passed in review, in more or less detail, the
main facts on which the theory of "the origin of species by
means of natural selection " is founded. In future chapters
we shall have to deal mainly with the application of the theory
to explain the varied and complex phenomena presented by the
organic world ; and, also, to discuss some of the theories put
forth by modern writers, either as being more fundamental than

122 DARWINISM

that of Darwin or as supplementary to it. Before doing this,
however, it will be well briefly to summarise the facts and
arguments already set forth, because it is only by a clear
comprehension of these that the full importance of the theory
can be appreciated and its further applications understood.

The theory itself is exceedingly simple, and the facts on
which it rests — though excessively numerous individually, and
coextensive with the entire organic world — yet come under a
few simple and easily understood classes. These facts are, —
first, the enormous powers of increase in geometrical progres-
sion possessed by all organisms, and the inevitable struggle for
existence among them ; and, in the second place, the occurrence
of much individual variation combined with the hereditary
transmission of such variations. From these two great classes
of facts, which are universal and indisputable, there necessarily
arises, as Darwin termed it, the " preservation of favoured races
in the struggle for life," the continuous action of which, under
the ever-changing conditions both of the inorganic and organic
universe, necessarily leads to the formation or development of
new species.

But, although this general statement is complete and indis-
putable, yet to see its applications under all the complex
conditions that actually occur in nature, it is necessary always
to bear in mind the tremendous power and universality of the
agencies at work. We must never for an instant lose sight
of the fact of the enormously rapid increase of all organisms,
which has been illustrated by actual cases, given in our second
chapter, no less than by calculations of the results of un-
checked increase for a few years. Then, never forgetting
that the animal and plant population of any country is, on
the whole, stationary, we must be always trying to realise the
ever-recurring destruction of the enormous annual increase,
and asking ourselves what determines, in each individual case,
the death of the many, the survival of the few. We must
think over all the causes of destruction to each organism, — to
the seed, the young shoot, the growing plant, the full-grown
tree, or shrub, or herb, and again the fruit and seed ; and among
animals, to the egg or new-born young, to the youthful, and
to the adults. Then, we must always bear in mind that what
goes on in the case of the individual or family group we may

v NATURAL SELECTION 123

observe or think of, goes on also among the millions and
scores of millions of individuals which are comprised in almost
every species ;: and must get rid of the idea that chance
determines which shall live and which die. For, although in
many individual cases death may be due to chance rather
than to any inferiority in those which die first, yet we cannot
possibly believe that this can be the case on the large scale
on which nature works. A plant, for instance, cannot be in-
creased unless there are suitable vacant places its seeds can
grow in, or stations where it can overcome other less vigorous
and healthy plants. The seeds of all plants, by their varied
modes of dispersal, may be said to be seeking out such places
in which to grow ; and we cannot doubt that, in the long run,
those individuals whose seeds are the most numerous, have the
greatest powers of dispersal, and the greatest vigour of growth,
will leave more descendants than the individuals of the same
species which are inferior in all these respects, although now
and then some seed of an inferior individual may chance to be
carried to a spot where it can grow and survive. The same
rule will apply to every period of life and to every danger to
which plants or animals are exposed. The best organised, or
the most healthy, or the most active, or the best protected, or
the most intelligent, will inevitably, in the long run, gain an
advantage over those which are inferior in these qualities ;
that is, the fittest will survive, the fittest being, in each particular
case, those which are superior in the special qualities on
which safety depends. At one period of life, or to escape one
kind of danger, concealment may be necessary ; at another
time, to escape another danger, swiftness ; at another, intel-
ligence or cunning ; at another, the power to endure rain or
cold or hunger ; and those which possess all these faculties in
the fullest perfection will generally survive.

Having fully grasped these facts in all their fulness and
in their endless and complex results, we have next to consider
the phenomena of variation, discussed in the third and fourth
chapters ; and it is here that perhaps the greatest difficulty will
be felt in appreciating the full importance of the evidence as set
forth. It has been so generally the practice to speak of
variation as something exceptional and comparatively rare — -as
an abnormal deviation from the uniformity and stability of the

124 DARWINISM

characters of a species — and so few even among naturalists
have ever compared, accurately, considerable numbers of
individuals, that the conception of variability as a general
characteristic of all dominant and widespread species, large in
its amount and affecting, not a few, but considerable masses of
the individuals which make up the species, will be to many
entirely new. Equally important is the fact that the vari-
ability extends to every organ and every structure, external
and internal ; while perhaps most important of all is the
independent variability of these several parts, each one vary-
ing without any constant or even usual dependence on, or
correlation with, other parts. No doubt there is some such
correlation in the differences that exist between species and
species — more developed wings usually accompanying smaller
feet and vice versa — but this is, generally, a useful adaptation
which has been brought about by natural selection, and does
not apply to the individual variability which occurs within
the species.

It is because these facts of variation are so important and
so little understood, that they have been discussed in what
will seem to some readers wearisome and unnecessary detail.
Many naturalists, however, will hold that even more evidence
is required; and more, to almost any amount, could easily
have been given. The character and variety of that already
adduced will, however, I trust, convince most readers that
the facts are as stated ; while they have been drawn from
a sufficiently wide area to indicate a general principle through-
out nature.

If, now, we fully realise these facts of variation, along with
those of rapid multiplication and the struggle for existence,
most of the difficulties in the way of comprehending how species
have originated through natural selection will disappear. For
whenever, through changes of climate, or of altitude, or of
the nature of the soil, or of the area of the country, any
species are exposed to new dangers, and have to maintain
themselves and provide for the safety of their offspring under
new and more arduous conditions, then, in the variability of
all parts, organs, and structures, no less than of habits and
intelligence, we have the means of producing modifications
which will certainly bring the species into harmony with its

v NATURAL SELECTION 125

new conditions. And if Ave remember that all such physical
changes are slow and gradual in their operation, we shall see
that the amount of variation which we know occurs in every
new generation will be quite sufficient to enable modification
and adaptation to go on at the same rate. Mr. Darwin
was rather inclined to exaggerate the necessary slowness of
the action of natural selection ; but with the knowledge we
now possess of the great amount and range of individual
variation, there seems no difficulty in an amount of change,
quite equivalent to that which usually distinguishes allied
species, sometimes taking place in less than a century, should
any rapid change of conditions necessitate an equally rapid
adaptation. This may often have occurred, either to im-
migrants into a new land, or to residents whose country has
been cut off by subsidence from a larger and more varied
area over which they had formerly roamed. When no change
of conditions occurs, species may remain unchanged for very long
periods, and thus produce that appearance of stability of species
which is even now often adduced as an argument against
evolution by natural selection, but which is really quite in
harmony with it.

On the principles, and by the light of the facts, now briefly
summarised, we have been able, in the present chapter, to
indicate how natural selection acts, how divergence of char-
acter is set up, how adaptation to conditions at various periods
of life has been effected, how it is that low forms of life
continue to exist, what kind of circumstances are most
favourable to the formation of new species, and, lastly, to
what extent the advance of organisation to higher types is
produced by natural selection. We will now pass on to con-
sider some of the more important objections and difficulties
which have been advanced by eminent naturalists.

CHAPTER VI

DIFFICULTIES AND OBJECTIONS

Difficulty as to sraallness of variations — As to the right variations occur-
ring when required — The beginnings of important organs — The mam-
mary glands — The eyes of flatfish — Origin of the eye — Useless or
non-adaptive characters — Recent extension of the region of utility in
plants — The same in animals — Uses of tails — Of the horns of deer —
Of the scale-ornamentation of reptiles — Instability of non-adaptive
characters — Delbceuf's law — No "specific" character proved to be
useless — The swamping effects of intercrossing — Isolation as prevent-
ing intercrossing— Gulick on the effects of isolation — Cases in which
isolation is ineffective.

In the present chapter I propose to discuss the more obvious
and often repeated objections to Darwin's theory, and to show
how far they affect its character as a true and sufficient
explanation of the origin of species. The more recondite
difficulties, affecting such fundamental questions as the causes
and laws of variability, will be left for a future chapter, after
we have become better acquainted with the applications of the
theory to the more important adaptations and correlations of
animal and plant life.

One of the earliest and most often repeated objections was,
that it was difficult " to imagine a reason why variations tend-
ing in an infinitesimal degree in any special direction should
be preserved," or to believe that the complex adaptation of
living organisms could have been produced " by infinitesimal
beginnings." Now this term " infinitesimal," used by a well-
known early critic of the Origin of Species, was never made use
of by Darwin himself, who spoke only of variations being
"slight," and of the "small amount" of the variations that might
be selected. Even in using these terms he undoubtedly afforded

chap, vi DIFFICULTIES AND OBJECTIONS • 127

grounds for the objection above made, that such small and
slight variations could be of no real use, and would not
determine the survival of the individuals possessing them. We
have seen, however, in our third chapter, that even Darwin's
terms were hardly justified; and that the variability of many im-
portant species is of considerable amount, and may very often
be properly described as large. As this is found to be the
case both in animals and plants, and in all their chief groups
and subdivisions, and also to apply to all the separate parts
and organs that have been compared, we must take it as
proved that the average amount of variability presents no
difficulty whatever in the way of the action of natural selection.
It may be here mentioned that, up to the time of the prepara-
tion of the last edition of The Origin of Species, Darwin had
not seen the work of Mr. J. A. Allen of Harvard University
(then only just published), which gave us the first body of accu-
rate comparisons and measurements demonstrating this large
amount of variability. Since then evidence of this nature
has been accumulating, and we are, therefore, now in a far
better position to appreciate the facilities for natural selection,
in this respect, than was Mr. Darwin himself.

Another objection of a similar nature is, that the chances
are immensely against the right variation or combination of
variations occurring just when required ; and further, that no
variation can be perpetuated that is not accompanied by
several concomitant variations of dependent parts — greater
length of a wing in a bird, for example, would be of little use
if unaccompanied by increased volume or contractility of the
muscles which move it. This objection seemed a very strong
one so long as it was supposed that variations occurred singly
and at considerable intervals ; but it ceases to have any weight
now we know that they occur simultaneously in various parts
of the organism, and also in a large proportion of the in-
dividuals which make up the species. A considerable number
of individuals will, therefore, every year possess the required
combination of characters ; and it may also be considered
probable that when the two characters are such that they
always act together, there will be such a correlation between
them that they will frequently vary together. But there is
another consideration that seems to show that this coincident

128 DARWINISM

variation is not essential. All animals in a state of nature
are kept, by the constant struggle for existence and the
survival of the fittest, in such a state of perfect health and
usually superabundant vigour, that in all ordinary circumstances
they possess a surplus power in every important organ — a
surplus only drawn upon in cases of the direst necessity when
their very existence is at stake. It follows, therefore, that
any additional power given to one of the component parts of
an organ must be useful — an increase, for example, either in
the wing muscles or in the form or length of the wing might give
some increased powers of flight ; and thus alternate variations —
in one generation in the muscles, in another generation in the
wing itself — might be as effective in permanently improving the
powers of flight as coincident variations at longer intervals.
On either supposition, however, this objection appears to have
little weight if we take into consideration the large amount of
coincident variability that has been shown to exist.

The Beginnings of Important Organs.

We iioav come to an objection which has perhaps been
more frequently urged than any other, and which Darwin
himself felt to have much weight — the first beginnings of im-
portant organs, such, for example, as wings, eyes, mammary
glands, and numerous other structures. It is urged, that it
is almost impossible to conceive how the first rudiments of
these could have been of any use, and, if not of use they could
not have been preserved and further developed by natural
selection.

Now, the first remark to be made on objections of this
nature is, that they are really outside the question of the
origin of all existing species from allied species not very far
removed from them, which is all that Darwin undertook to
prove by means of his theory. Organs and structures such as
those above mentioned all date back to a very remote past,
when the world and its inhabitants were both very different
from what they are now. To ask of a new theory that it
shall reveal to us exactly what took place in remote geological
epochs, and how it took place, is unreasonable. The most
that should be asked is, that some probable or possible mode of
origination should be pointed out in some at least of these

vi DIFFICULTIES AND OBJECTIONS 129

difficult cases, and this Mr. Darwin lias done. One or two of
these may he hriefly given here, hut the whole series should
be carefully read by any one who wishes to see how many
curious facts and observations have been required in order to
elucidate them ; whence we may conclude that further know-
ledge will probably throw light on any difficulties that still
remain.1

In the case of the mammary glands Mr. Darwin remarks
that it is admitted that the ancestral mammals were allied to
the marsupials. Now in the very earliest mammals, almost
before they really deserved that name, the young may have
been nourished by a fluid secreted by the interior surface of
the marsupial sack, as is believed to be the case with the
fish (Hippocampus) whose eggs are hatched within a some-
what similar sack. This being the case, those individuals
which secreted a more nutritious fluid, and those whose
young were able to obtain and swallow a more constant supply
by suction, would be more likely to live and come to a healthy
maturity, and would therefore be preserved by natural selec-
tion.

In another case which has been adduced as one of special
difficulty, a more complete explanation . is given. Soles,
turbots, and other flatfish are, as is Avell known, unsym-
metrical. They live and move on their sides, the under side
being usually differently coloured from that which is kept
uppermost. Noav the eyes of these fish are curiously distorted
in order that both eyes may be on the upper side, where alone
they would be of any use. It was objected by Mr. Mivart
that a sudden transformation of the eye from one side to the
other was inconceivable, while, if the transit were gradual
the first step could be of no use, since this would not remove
the eye from the lower side. But, as Mr. Darwin shows by
reference to the researches of Malm and others, the young of
these fish are quite symmetrical, and during their growth
exhibit to us the whole process of change. This begins by
the fish (owing to the increasing depth of the body) being un-
able to maintain the vertical position, so that it falls on one side.
It then twists the lower eye as much as possible towards the
upper side ; and, the whole bony structure of the head being at
1 See Origin of Species, pp. 176-198.
K

130 DARWINISM chap.

this time soft and flexible, the constant repetition of this effort
causes the eye gradually to move round the head till it comes
to the upper side. Now if we suppose this process, which in
the young is completed in a few days or weeks, to have been
spread over thousands of generations during the development of
these fish, those usually surviving whose eyes retained more
and more of the position into which the young fish tried to
twist them, the change becomes intelligible ; though it still
remains one of the most extraordinary cases of degeneration, by
which symmetry — which is so universal a characteristic of
the higher animals — is lost, in order that the creature may be
adapted to a new mode of life, whereby it is enabled the better
to escajDe danger and continue its existence.

The most difficult case of all, that of the eye — the thought
of which even to the last, Mr. Darwin says, " gave him a cold
shiver" — is nevertheless shown to be not unintelligible;
granting of course the sensitiveness to light of some forms of
nervous tissue. For he shows that there are, in several of the
lower animals, rudiments of eyes, consisting merely of pigment
cells covered with a translucent skin, which may possibly
serve to distinguish light from darkness, but nothing more.
Then we have an optic nerve and pigment cells ; then we
find a hollow filled with gelatinous substance of a convex
form — the first rudiment of a lens. Many of the succeeding
stej^s are lost, as would necessarily be the case, owing to the
great advantage of each modification which gave increased
distinctness of vision, the creatures possessing it inevitably
surviving, while those below them became extinct. But we
can well understand how, after the first step was taken, every
variation tending to more complete vision Avould be preserved
till we reached the perfect eye of birds and mammals. Even
this, as we know, is not absolutely, but only relatively, perfect.
Neither the chromatic nor the spherical aberration is absolutely
corrected ; while long- and short- sightedness, and the various
diseases and imperfections to which the eye is liable, may be
looked upon as relics of the imperfect condition from which
the eye has been raised by variation and natural selection.

These few examples of difficulties as to the origin of remark-
able or complex organs must suffice here ; but the reader who
wishes further information on the matter may study carefully

vi DIFFICULTIES AND OBJECTIONS 131

the whole of the sixth and seventh chapters of the last edition
of The Origin of Species, in which these and many other cases
are discussed in considerable detail.

Useless or non-adaptive Characters.

Many naturalists seem to be of opinion that a considerable
number of the characters which distinguish species are of no
service whatever to their possessors, and therefore cannot have
been produced or increased by natural selection. Professors
Bronn and Broca have urged this objection on the continent.
In America, Dr. Cope, the well-known palaeontologist, has long-
since put forth the same objection, declaring that non-adaptive
characters are as numerous as those which are adaptive ; but
he differs completely from most who hold the same general
opinion in considering that they occur chiefly " in the
characters of the classes, orders, families, and other higher
groups;" and the objection, therefore, is quite distinct from
that in which it is urged that " specific characters " are mostly
useless. More recently, Professor G. J. Romanes has urged this
difficulty in his paper on " Physiological Selection " (Journ.
Linn. Soc, vol. xix. pp. 338, 344). He says that the characters
" which serve to distinguish allied species are frequently, if
not usually, of a kind with which natural selection can have
had nothing to do," being without any utilitarian significance.
Again he speaks of " the enormous number," and further on of
" the innumerable multitude " of specific peculiarities which
are useless ; and he finally declares that the question needs no
further arguing, " because in the later editions of his works
Mr. Darwin freely acknowledges that a large proportion of
specific distinctions must be conceded to be useless to the
species presenting them."

I have looked in vain in Mr. Darwin's works to find any
such acknowledgment, and I think Mr. Romanes has not
sufficiently distinguished between " useless characters " and
"useless specific distinctions." On referring to all the passages
indicated by him I find that, in regard to specific characters,
Mr. Darwin is very cautious in admitting inutility. His
most pronounced "admissions" on this question are the follow-
ing: "But when, from the nature of the organism and of
the conditions, modifications have been induced which are

132 DARWINISM chap.

unimportant for the welfare of the species, they may be, and
apparently often have been, transmitted in nearly the same
state to numerous, otherwise modified, descendants" {Origin, p.
175). The words I have here italicised clearly show that
such characters -are usually not " specific," in the sense that
they are such as distinguish species from each other, but are
found in numerous allied species. Again : " Thus a large
yet undefined extension may safely be given to the direct and
indirect results of natural selection ; but I now admit, after
reading the essay of Nageli on plants, and the remarks by
various authors with respect to animals, more especially those
recently made by Professor Broca, that in the earlier editions
of my Origin of Species I perhaps attributed too much to the
action of natural selection or the survival of the fittest. I
have altered the fifth edition of the Origin so as to confine my
remarks to adaptive changes of structure, but I am convinced,
from the light gained during even the last few years, that very
many structures which now appear to us useless, will hereafter be
proved to be useful, and will therefore come within the range of
natural selection. Nevertheless I did not formerly consider
sufficiently the existence of structures which, as far as we can at
present judge, are neither beneficial nor injurious ; and this I
believe to be one of the greatest oversights as yet detected in
my work." Now it is to be remarked that neither in these
passages nor in any of the other less distinct expressions of
opinion on this question, does Darwin ever admit that "specific
characters " — that is, the particular characters which serve to
distinguish one species from another — are ever useless, much
less that " a large proportion of them " are so, as Mr. Romanes
makes him "freely acknowledge." On the other hand, in
the passage which I have italicised he strongly expresses his
view that much of what we suppose to be useless is due to
our ignorance ; and as I hold myself that, as regards many of
the supposed useless characters, this is the true explanation,
it may be well to give a brief sketch of the progress of know-
ledge in transferring characters from the one category to
the other.

We have only to go back a single generation, and not even
the most acute botanist could have suggested a reasonable use,
for each species of plant, of the infinitely varied forms, sizes,

vi DIFFICULTIES AND OBJECTION'S 133

and colours of the flowers, the shapes and arrangement of the
leaves, and the numerous other external characters of the
whole plant. But since Mr. Darwin showed that plants
gained both in vigour and in fertility by being crossed with
other individuals of the same species, and that this crossing
,was usually effected by insects which, in search of nectar or
pollen, carried the pollen from one plant to the flowers of
another plant, almost every detail is found to have a purpose
and a use. The shape, the size, and the colour of the petals,
even the streaks and spots with which they are adorned, the
position in which they stand, the movements of the stamens
and pistil at various times, especially at the period of, and
just after, fertilisation, have been proved to be strictly
adaptive in so many cases that botanists now believe that all
the external characters of flowers either are or have been of
use to the species.

It has also been shown, by Kerner and other botanists,
that another set of characteristics have relation to the pre-
vention of ants, slugs, and other animals from reaching the
flowers, because these creatures would devour or injure
them without effecting fertilisation. The spines, hairs,
or sticky glands on the stem or flower-stalk, the curious
hairs or processes shutting up the flower, or sometimes
even the extreme smoothness and polish of the outside of
the petals so that few insects can hang to the part, have
been shown to be related to the possible intrusion of
these "unbidden guests."1 And, still more recently, attempts
have been made by Grant Allen and Sir John Lubbock
to account for the innumerable forms, textures, and groupings
of leaves, by their relation to the needs of the plants
themselves ; and there can be little doubt that these
attempts will be ultimately successful. Again, just as floAvers
have been adapted to secure fertilisation or cross-fertilisation,
fruits have been developed to assist in the dispersal of seeds ;
and their forms, sizes, juices, and colours can be shown to be
specially adapted to secure such dispersal by the agency of
birds and mammals ; while the same end is secured in other

1 See Kerner's Flowers and their Unhidden Guests for numerous other
structures and peculiarities of plants which are shown to be adaptive and
useful.

134 DARWINISM

cases by downy seeds to be wafted through the air, or by
hooked or sticky seed-vessels to be carried away, attached to
skin, wool, or feathers.

Here, then, we have an enormous extension of the region of
utility in the vegetable kingdom, and one, moreover, which
includes almost all the specific characters of plants. For the
species of plants are usually characterised either by differences
in the form, size, and colour of the flowers, or of the fruits ;
or, by peculiarities in the shape, size, dentation, or arrange-
ment of the leaves ; or by peculiarities in the spines, hairs, or
down with which various parts of the plant are clothed. In
the case of plants it must certainly be admitted that " specific "
characters are pre-eminently adaptive ; and though there may
be some which are not so, yet all those referred to by Darwin
as having been adduced by various botanists as useless, either
pertain to genera or higher groups, or are found in some
plants of a species only — that is, are individual variations not
specific characters.

In the case of animals, the most recent wide extension of
the sphere of utility has been in the matter of their colours
and markings. It was of course always known that certain
creatures gained protection by their resemblance to their
normal surroundings, as in the case of white arctic animals,
the yellow or brown tints of those living in deserts, and the
green hues of many birds and insects surrounded by tropical
vegetation. But of late years these cases have been greatly
increased both in number and variety, especially in regard to
those which closely imitate special objects among which they
live ; and there are other kinds of coloration which long
appeared to have no use. Large numbers of animals, more
especially insects, are gaudily coloured, either with vivid hues
or with striking patterns, so as to be very easily seen. Now
it has been found, that in almost all these cases the creatures
possess some special quality which prevents their being
attacked by the enemies of their kind whenever the
peculiarity is known ; and the brilliant or conspicuous colours
or markings serve as a warning or signal flag against attack.
Large numbers of insects thus coloured are nauseous and
inedible ; others, like wasps and bees, have stings ; others are
too hard to be eaten by small birds ; while snakes with

vr DIFFICULTIES AND OBJECTIONS 135

poisonous fangs often have some characteristic either of
rattle, hood, or unusual colour, which indicates that they had
better be left alone.

But there is yet another form of coloration, which
consists in special markings — bands, spots, or patches of white,
or of bright colour, which vary in every species, and are often
concealed when the creature is at rest but displayed when in
motion, — as in the case of the bands and spots so frequent on
the wings and tails of birds. Now these specific markings
are believed, with good reason, to serve the purpose of enabling
each species to be quickly recognised, even at a distance, by
its fellows, especially the parents by their young and the two
sexes by each other ; and this recognition must often be an
important factor in securing the safety of individuals, and
therefore the wellbeing and continuance of the species.
These interesting peculiarities will be more fully described in
a future chapter, but they are briefly referred to here in
order to show that the most common of all the characters by
which species are distinguished from each other — their colours
and markings — can be shown to be adaptive or utilitarian in
their nature.

But besides colour there are almost always some structural
characters which distinguish species from species, and, as re-
gards many of these also, an adaptive character can be often
discerned. In birds, for instance, we have differences in the
size or shape of the bill or the feet, in the length of the wing
or the tail, and in the proportions of the several feathers of
which these organs are composed. All these differences in
the organs on which the very existence of birds depends,
which determine the character of flight, facility for running
or climbing, for inhabiting chiefly the ground or trees, and
the kind of food that can be most easily obtained for
themselves and their offspring, must surely be in the highest
degree utilitarian ; although in each individual case we, in our
ignorance of the minutiae of their life-history, may be quite
unable to see the use. In mammalia specific differences other
than colour usually consist in the length or shape of the ears
and tail, in the proportions of the limbs, or in the length
and quality of the hair on different parts of the body. As
regards the ears and tail, one of the objections by Professor

136 DARWINISM

Bronn relates to this very point. He states that the length of
these organs differ in the various species of hares and of mice,
and he considers that this difference can be of no service
whatever to their possessors. But to this objection Darwin
replies, that it has been shown by Dr. Schobl that the ears of
mice " are supplied in an extraordinary manner with nerves,
so that they no doubt serve as tactile organs." Hence, when
we consider the life of mice, either nocturnal or seeking their
food in dark and confined places, the length of the ears
may be in each case adapted to the particular habits and
surroundings of the species. Again, the tail, in the larger
mammals, often serves the purpose of driving off flies and
other insects from the body ; and when we consider in how
many parts of the world flies are injurious or even fatal to
large mammals, we see that the peculiar characteristics of this
organ may in each case have been adapted to its requirements
in the particular area where the species was developed. The
tail is also believed to have some use as a balancing organ,
which assists an animal to turn easily and rapidly, much as
our arms are used when running ; while in whole groups it is
a prehensile organ, and has become modified in accordance
with the habits and needs of each species. In the case of
mice it is thus used by the young. Darwin informs us that
the late Professor Henslow kept some harvest-mice in con-
finement, and observed that they frequently curled their tails
round the branches of a bush placed in the cage, and thus
aided themselves in climbing ; while Dr. Giinther has actually
seen a mouse suspend itself by the tail {Origin, p. 189).

Again, Mr. Lawson Tait has called attention to the use of
the tail in the cat, squirrel, yak, and many other animals as
a means of preserving the heat of the body during the
nocturnal and the winter sleep. He says, that in cold weather
animals with long or bushy tails will be found lying curled up,
with their tails carefully laid over their feet like a rug, and
with their noses buried in the fur of the tail, which is thus
used exactly in the same way and for the same purpose as we
use respirators.1

Another illustration is furnished by the horns of deer
which, especially when very large, have been supposed to be
1 Nature, vol. xx. p. C03.

vi DIFFICULTIES AND OBJECTIONS 137

a danger to the animal in passing rapidly through dense
thickets. But Sir James Hector states, that the wapiti, in
North America, throws back its head, thus placing the horns
along the sides of the back, and is then enabled to rush
through the thickest forest with great rapidity. The brow-
antlers protect the face and eyes, while the widely spreading
horns prevent injury to the neck or flanks. Thus an organ
which was certainly developed as a sexual weapon, has been
so guided and modified during its increase in size as to be of
use in other ways. A similar use of the antlers of deer
has been observed in India.1

The various classes of facts now referred to serve to show
us that, in the case of the two higher groups— mammalia
and birds — almost all the characters by which species are
distinguished from each other are, or may be, adaptive. It is
these two- classes of animals which have been most studied
and whose life-histories are supposed to be most fully known,
yet even here the assertion of inutility, by an eminent
naturalist, in the case of two important organs, has been
sufficiently met by minute details either in the anatomy or in
the habits of the groups referred to. Such a fact as this,
together with the extensive series of characters already
enumerated which have been of late years transferred from
the "useless" to the "useful" class, should convince us, that
the assertion of " inutility " in the case of any organ or
peculiarity which is not a rudiment or a correlation, is not,
and can never be, the statement of a fact, but merely an
expression of our ignorance of its purpose or origin.2

1 Nature, vol. xxxviii. p. 328.

2 A very remarkable illustration of function in an apparently useless
ornament is given by Semper. He says, ' ' It is known that the skin of
reptiles encloses the body with scales. These scales are distinguished by
very various sculpturings, highly characteristic of the different species.
Irrespective of their systematic significance they appear to be of no value in
the life of the animal ; indeed, they are viewed as ornamental without regard
to the fact that they are microscopic and much too delicate to be visible to
other animals of their own species. It might, therefore, seem hopeless to show
the necessity for their existence on Darwinian principles, and to prove that
they are physiologically active organs. Nevertheless, recent investigations on
this point have furnished evidence that this is possible.

" It is known that many reptiles, and above all the snakes, cast off the
whole skin at once, whereas human beings do so by degrees. If by any
accident they are prevented doing so, they infallibly die, because the old

138 DARWINISM

Instability of Non-adaptive Characters.

One very weighty objection to the theory that specific
characters can ever be wholly useless (or wholly uncon-
nected with useful organs by correlation of growth) appears
to have been overlooked by those who have maintained
the frequency of such characters, and that is, their almost
necessary instability. Darwin has remarked on the extreme
variability of secondary sexual characters — such as the horns,
crests, plumes, etc., which are found in males only,^ — the
reason being, that, although of some use, they are not
of such direct and vital importance as those adaptive
characters on which the wellbeing and very existence of the
animals depend. But in the case of wholly useless structures,

skin has grown so tough and hard that it hinders the increase in volume
which is inseparable from the growth of the animal. The casting of the
skin is induced by the formation on the surface of the inner epidermis, of a
layer of very fine and equally distributed hairs, which evidently serve the
purpose of mechanically raising the old skin by their rigidity and position.
These hairs then may be designated as casting hairs. That they are destined
and calculated for this end is evident to me from the fact established by Dr.
Braun, that the casting of the shells of the river cray-fish is induced in exactly
the same manner by the formation of a coating of hairs which mechanically
loosens the old skin or shell from the new. Now the researches of Braun and
Cartier have shown that these casting hairs — which serve the same purpose in
two groups of animals so far apart in the systematic scale — after the casting,
are partly transformed into the concentric stripes, sharp spikes, ridges, or
warts which ornament the outer edges of the skin-scales of reptiles or the
carapace of crabs." x Professor Semper adds that this example, with many
others that might be quoted, shows that we need not abandon the hope of
explaining morphological characters on Darwinian principles, although their
nature is often difficult to understand.

During a recent discussion of this question in the pages of Nature, Mr.
St. George Mivart adduces several examples of what he deems useless specific
characters. Among them are the aborted index finger of the lemurine Potto,
and the thumbless hands of Colobus and Ateles, the " life-saving action " of
either of which he thinks incredible. These cases suggest two remarks. In
the first place, they involve generic, not specific, characters ; and the three
genera adduced are somewhat isolated, implying considerable antiquity and
the extinction of many allied forms. This is important, because it affords
ample time for great changes of conditions since the structures in question
originated ; and without a knowledge of these changes we can never safely
assert that any detail of structure could not have been useful. In the second
place, all three are cases of aborted or rudimentary organs ; and these are
admitted to be explained by non-use, leading to diminution of size, a further
reduction being brought about by the action of the principle of economy

1 The Natural Conditions of Existence as they affect Animal Life, p. 19.

vi DIFFICULTIES AND OBJECTIONS 139

which are not rudiments of once useful organs, we cannot see
what there is to ensure any amount of constancy or stability.
One of the cases on which Mr. Romanes lays great stress in
his paper on "Physiological Selection" (Journ. Linn. Soc, vol. xix.
p. 384) is that of the fleshy appendages on the corners of the
jaw of Normandy pigs and of some other breeds. But it is
expressly stated that they are not constant ; they appear
"frequently," or "occasionally," they are "not strictly
inherited, for they occur or fail in animals of the same litter ;"
and they are not always symmetrical, sometimes appearing
on one side of the face alone. Now whatever may be the
cause or explanation of these anomalous appendages they
cannot be classed with " specific characters," the most
essential features of which are, that they are symmetrical,

of growth. J3ut, when so reduced, the rudiment might he inconvenient or even
hurtful, and then natural selection would aid in its complete abortion ; in
other words, the abortion of the part would be useful, and would therefore be
subject to the law of survival of the fittest. The genera Ateles and Colobus
are two of the most purely arboreal types of monkeys, and it is not difficult
to conceive that the constant use of the elongated fingers for climbing from
tree to tree, and catching on to branches while making great leaps, might
require all the nervous energy and muscular growth to be directed to the
fingers, the small thumb remaining useless. The case of the Potto is more
difficult, both because it is, presumably, a more ancient type, and its actual life-
history and habits are completely unknown. These cases are, therefore, not
at all to the point as proving that positive specific characters — not mere
rudiments characterising whole genera — are in any case useless.

Mr. Mivart further objects to the alleged rigidity of the action of natural
selection, because wounded or malformed animals have been found which had
evidently lived a considerable time in their imperfect condition. But this
simply proves that they were living under a temporarily favourable environ-
ment, and that the real struggle for existence, in their case, had not yet
taken place. We must surely admit that, when the pinch came, and when
perfectly formed stoats were dying for want of food, the one-footed animal,
referred to by Mr. Mivart, would be among the first to succumb ; and the
same remark will apply to his abnormally toothed hares and rheumatic
monkeys, which might, nevertheless, get on very well under favourable
conditions. The struggle for existence, under which all animals and plants
have been developed, is intermittent, and exceedingly irregular in its incidence
and severity. It is most severe and fatal to the young ; but when an animal
has once reached maturity, and especially when it has gained experience by
several years of an eventful existence, it may be able to maintain itself under
conditions which would be fatal to a young and inexperienced creature of the
same species. The examples adduced by Mr. Mivart do not, therefore, in
any way impugn the hardness of nature as a taskmaster, or the extreme
severity of the recurring struggle for existence. 1

1 See Nature, vol. xxxix. p. 127.

140 DARWINISM

that they are inherited, and that they are constant. Ad-
mitting that this peculiar appendage is (as Mr. Romanes says
rather confidently, " we happen to know it to be ") wholly
useless and meaningless, the fact would be rather an argument
against specific characters being also meaningless, because the
latter never have the characteristics which this particular
variation possesses.

These useless or non-adaptive characters are, apparently, of
the same nature as the " sports " that arise in our domestic
productions, but which, as Mr. Darwin says, without the aid
of selection would soon disappear ; while some of them may
be correlations with other characters which are or have been
useful. Some of these correlations are very curious. Mr.
Tegetmeier informed Mr. Darwin that the young of white, yellow,
or dun-coloured pigeons are born almost naked, whereas other
coloured pigeons are born well clothed with down. Now, if
this difference occurred between wild species of different colours,
it might be said that the nakedness of the young could not be
of any use. But the colour with which it is correlated might,
as has been shown, be useful in many ways. The skin and its
various appendages, as horns, hoofs, hair, feathers, and teeth,
are homologous parts, and are subject to very strange correla-
tions of growth. In Paraguay, horses with curled hair occur,
and these always have hoofs exactly like those of a mule, while
the hair of the mane and tail is much shorter than usual.
Now, if any one of these characters were useful, the others
correlated with it might be themselves useless, but would still
be tolerably constant because dependent on a useful organ.
So the tusks and the bristles of the boar are correlated and
vary in development together, and the former only may be
useful, or both may be useful in unequal degrees.

The difficulty as to how individual differences or sports can
become fixed and perpetuated, if altogether useless, is evaded
by those who hold that such characters are exceedingly common.
Mr. Romanes says that, upon his theory of physiological selec-
tion, "it is quite intelligible that when a varietal form is
differentiated from its parent form by the bar of sterility, any
little meaningless peculiarities of structure or of instinct should
at first be allowed to arise, and that they should then be allowed
to perpetuate themselves by heredity," until they are finally

vt DIFFICULTIES AND OBJECTIONS 141

eliminated by disuse. But this is entirely begging the ques-
tion. Do meaningless peculiarities, which we admit often arise
as spontaneous variations, ever perpetuate themselves in all
the individuals constituting a variety or race, without selec-
tion either human or natural ? Such characters present them-
selves as unstable variations, and as such they remain, unless
preserved and accumulated by selection ; and they can there-
fore never become " specific " characters unless they are strictly
correlated with some useful and important peculiarities.

As bearing upon this question we may refer to what is
termed Delboeuf's law, which has been thus briefly stated by
Mr. Murphy in his work on Habit and Intelligence, p.
241.

" If, in any species, a number of individuals, bearing a
ratio not infinitely small to the entire number of births, are in
every generation born with a particular variation which is
neither beneficial nor injurious, and if it is not counteracted by
reversion, then the proportion of the new variety to the original
form will increase till it approaches indefinitely near to
equality."

It is not impossible that some definite Varieties, such as the
melanic form of the jaguar and the bridled variety of the guille-
mot are due to this cause ; but from their very nature such
varieties are unstable, and are continually reproduced in
varying proportions from the parent forms. They can,
therefore, never constitute species unless the variation in
question becomes beneficial, when it will be fixed by natural
selection. Darwin, it is true, says — " There can be little
doubt that the tendency to vary in the same manner has often
been so strong that all the individuals of the same species
have been similarly modified without the aid of any form of
selection." 1 But no proof whatever is offered of this state-
ment, and it is so entirely opposed to all we know of the facts
of variation as given by Darwin himself, that the important
word " all " is probably an oversight.

On the whole, then, I submit, not only has it not been

proved that an " enormous number of specific peculiarities "

are useless, and that, as a logical result, natural selection is

" not a theory of the origin of species," but only of the origin

1 Origin of Species, p. 72.

142 DARWINISM

of adaptations which are usually common to many species, or,
more commonly, to genera and families ; but, I urge further,
it has not even been proved that any truly " specific "
characters — those which either singly or in combination dis-
tinguish each species from its nearest allies — are entirely un-
adaptive, useless, and meaningless ; while a great body of facts
on the one hand, and some weighty arguments on the other,
alike prove that specific characters have been, and could only
have been, developed and fixed by natural selection because of
their utility. We may admit, that among the great number of
variations and sports which continually arise many are altogether
useless without being hurtful ; but no cause or influence has
been adduced adequate to render such characters fixed and
constant throughout the vast number of individuals which con-
stitute any of the more dominant species.1

The Swamping Effects of Intercrossing.

This supposed insuperable difficulty was first advanced in
an article in the North British Review in 1867, and much
attention has been attracted to it by the acknowledgment of
Mr. Darwin that it "proved to him that "single variations,"
or what are usually termed " sports," could very rarely, if ever,
be perpetuated in a state of nature, as he had at first thought
might occasionally be the case.. But he had always considered
that the chief part, and latterly the whole, of the materials
with which natural selection Avorks, was afforded by individual
variations, or that amount of ever fluctuating variability which
exists in all organisms and in all their parts. Other writers
have urged the same objection, even as against individual
variability, apparently in total ignorance of its amount and
range ; and quite recently Professor G. J. Romanes has adduced

1 Darwin's latest expression of opinion on this question is interesting, since
it shows that he was inclined to return to his earlier view of the general, or
universal, utility of specific characters. In a letter to Semper (30th Nov.
1878) he writes : "As our knowledge advances, very slight differences, con-
sidered by systematists as of no importance in structure, are continually
found to be functionally important ; and I have been especially struck with
this fact in the case of plants, to which my observations have, of late years,
been confined. Therefore it seems to me rather rash to consider slight
differences between representative species, for instance, those inhabiting the
different islands of the same archipelago, as of no functional importance, and
as not in any way due to natural selection " {Life of Darwin, vol. iii. p. 161).

vr DIFFICULTIES AND OBJECTIONS 143

it as one of the difficulties which can alone be overcome by his
theory of physiological selection. He urges, that the same
variation does not occur simultaneously in a number of
individuals inhabiting the same area, and that it is mere
assumption to say it does ; while he admits that " if the
assumption were granted there would be an end of the present
difficulty ; for if a sufficient number of individuals were thus
simultaneously and similarly modified, there need be no longer
any danger of the variety becoming swamped by intercrossing."
I must again refer my readers to my third chapter for the
proof that such simultaneous variability is not an assumption
but a fact ; but, even admitting this to be proved, the problem
is not altogether solved, and there is so much misconception
regarding variation, and the actual process of the origin of
new species is so obscure, that some further discussion and
elucidation of the subject are desirable.

In one of the preliminary chapters of Mr. Seebohm's recent
work on the Charadriidce, he discusses the differentiation of
species ; and he expresses a rather widespread view among
naturalists when, speaking of the swamping effects of inter-
crossing, he adds : " This is unquestionably a very grave
difficulty, to my mind an absolutely fatal one, to the theory of
accidental variation." And in another passage he says : "The
simultaneous appearance, and its repetition in successive genera-
tions, of a beneficial variation, in a large number of individuals in
the same locality, cannot possibly be ascribed to chance." These
remarks appear to me to exhibit an entire misconception of the
facts of variation as they actually occur, and as they have been
utilised by natural selection in the modification of species. I
have already shown that every part of the organism, in common
species, does vary to a very considerable amount, in a large
number of individuals, and in the same locality ; the only point
that remains to be discussed is, whether any or most of these
variations are "beneficial." But every one of these variations
consists either in increase or diminution of size or power of the
organ or faculty that varies ; they can all be divided into a
more effective and a less effective group — that is, into one that
is more beneficial or less beneficial. If less size of body would
be beneficial, then, as half the variations in size are above and
half below the mean or existing standard of the species, there

144 DARWINISM

would be ample beneficial variations ; if a darker colour or
a longer beak or wing were required, there are always a con-
siderable number of individuals darker and lighter in colour
than the average, with longer or with shorter beaks and wings,
and thus the beneficial variation must always be present. And
so with every other part, organ, function, or habit ; because, as
variation, so far as we know, is and always must be in the two
directions of excess and defect in relation to the mean amount,
whichever kind of variation is wanted is always present in some
degree, and thus the difficulty as to " beneficial " variations
occurring, as if they were a special and rare class, falls to the
ground. No doubt some organs may vary in three or perhaps
more directions, as in the length, breadth, thickness, or curva-
ture of the bill. But these may be taken as separate varia-
tions, each of which again occurs as " more " or " less "; and thus
the " right " or " beneficial " or " useful " variation must always
be present so long as any variation at all occurs ; and it has not
yet been proved that in any large or dominant species, or in
any part, organ, or faculty of such species, there is no variation.
And even were such a case found it would prove nothing, so
long as in numerous other species variation Avas shown to exist ;
because we know that great numbers of species and groups
throughout all geological time have died out, leaving no
descendants ; and the obvious and sufficient explanation of this
fact is, that they did not vary enough at the time when varia-
tion was required to bring them into harmony with changed
conditions. The objection as to the " right " or " beneficial "
variation occurring when required, seems therefore to have no
weight in view of the actual facts of variation.

Isolation to prevent Intercrossing.

Most writers on the subject consider the isolation of a
portion of a species a very important factor in the formation
of new species, while others maintain it to be absolutely
essential. This latter view has arisen from an exaggerated
opinion as to the power of intercrossing to keep down any
vai'iety or incipient species, and merge it in the parent stock.
But it is evident that this can only occur with varieties which
are not useful, or Avhich, if useful, occur in very small
numbers ; and from this kind of variations it is clear that

vr DIFFICULTIES AND OBJECTIONS 145

new species do not arise. Complete isolation, as in an oceanic
island, will no doubt enable natural selection to act more
rapidly, for several reasons. In the first place, the absence of
competition will for some time allow the new immigrants to
increase rapidly till they reach the limits of subsistence.
They will then struggle among themselves, and by survival of
the fittest will quickly become adapted to the new conditions
of their environment. Organs which they formerly needed,
to defend themselves against, or to escape from, enemies,
being no longer required, would be encumbrances to be got
rid of, while the power of appropriating and digesting new
and varied food would rise in importance. Thus we may
explain the origin of so many flightless and rather bulky birds
in oceanic islands, as the dodo, the cassowary, and the extinct
moas. Again, while this process was going on, the complete
isolation would prevent its being checked by the immigration
of new competitors or enemies, which would be very likely to
occur in a continuous area ; while, of course, any intercrossing
with the original unmodified stock would be absolutely pre-
vented. If, now, before this change has gone very far, the
variety spreads into adjacent but rather distant islands, the
somewhat different conditions in each may lead to the
development of distinct forms constituting what are termed
representative species ; and these we find in the separate
islands of the Galapagos, the West Indies, and other ancient
groups of islands.

But such cases as these will only lead to the production of
a few peculiar species, descended from the original settlers
which happened to reach the islands ; whereas, in wide areas,
and in continents, we have variation and adaptation on a much
larger scale ; and, whenever important physical changes de-
mand them, with even greater rapidity. The far greater
complexity of the environment, together with the occurrence of
variations in constitution and habits, will often allow of
effective isolation, even here, producing all the results of actual
physical isolation. As we have already explained, one of the
most frequent modes in which natural selection acts is, by
adapting some individuals of a species to a somewhat different
mode of life, whereby they are able to seize upon unappropriated
places in nature, and in so doing they become practically

L

146 DARWINISM

isolated from their parent form. Let us suppose, for example,
that one portion of a species usually living in forests ranges
into the open plains, and finding abundance of food remains
there permanently. So long as the struggle for existence is
not exceptionally severe, these two portions of the species may
remain almost unchanged ; but suppose some fresh enemies are
attracted to the plains by the presence of these new immi-
grants, then variation and natural selection would lead to the
preservation of those individuals best able to cope with the
difficulty, and thus the open country form would become
modified into a marked variety or into a distinct species ;
and there would evidently be little chance of this modifica-
tion being checked by intercrossing with the parent form
which remained in the forest.

Another mode of isolation is brought about by the variety
— either owing to habits, climate, or constitutional change —
breeding at a slightly different time from the parent species.
This is known to produce complete isolation in the case of
many varieties of plants. Yet another mode of isolation is
brought about by changes of colour, and by the fact that in a
wild state animals of similar colours prefer to keep together
and refuse to pair with individuals of another colour. The
probable reason and utility of this habit will be explained
in another chapter, but the fact is well illustrated by the
cattle which have run wild in the Falkland Islands. These
are of several different colours, but each colour keeps in a
separate herd, often restricted to one part of the island ;
and one of these varieties — the mouse-coloured— is said to
breed a month earlier than the others ; so that if this
variety inhabited a larger area it might very soon be estab-
lished as a distinct race or species.1 Of course where the
change of habits or of station is still greater, as when a ter-
restrial animal becomes sub-aquatic, or when aquatic animals
come to live in tree -tops, as with the frogs and Crustacea
described at p. 11 8, the danger of intercrossing is reduced to
a minimum.

Several writers, however, not content with the indirect
effects of isolation here indicated, maintain that it is in itself
a cause of modification, and ultimately of the origination of
1 See Variation of Animals and Plants, vol. i. p. 86.

vi DIFFICULTIES AND OBJECTIONS 147

new species. This was the keynote of Mr. Vernon Wollaston's
essay on "Variation of Species," published in 1856, and it is
adopted by the Rev. J. G. Gulick in his paper on " Diversity
of Evolution under one Set of External Conditions " (Journ.
Linn. Soc. Zool., vol. xi. p. 496). The idea seems to be
that there is an inherent tendency to variation in certain
divergent lines, and that when one portion of a species is
isolated, even though under identical conditions, that tendency
sets up a divergence which carries that portion farther and
farther away from the original species. This view is held to
be supported by the case of the land shells of the Sandwich
Islands, which certainly present some very remarkable
phenomena. In this comparatively small area there are
about 300 species of land shells, almost all of which belong
to one family (or sub -family), the Achatinellidse, found
nowhere else in the world. ' The interesting point is the
extreme restriction of the species and varieties. The
average range of each species is only five or six miles,
while some are restricted to but one or two square miles,
and only a very few range over a whole island. The forest
region that extends over one of the mountain -ranges of the
island of Oahu, is about forty miles in length and five or six
miles in breadth ; and this small territory furnishes about
175 species, represented by 700 or 800 varieties. Mr.
Gulick states, that the vegetation of the different valleys
on the same side of this range is much the same, yet each
has a molluscan fauna differing in some degree from that
of any other. " We frequently find a genus represented
in several successive valleys by allied species, sometimes
feeding on the same, sometimes on different plants. In
every such case the valleys that are nearest to each other
furnish the most nearly allied forms ; and a full set of the
varieties of each species presents a minute gradation of forms
between the more divergent types found in the more widely
separated localities." He urges, that these constant differences
cannot be attributed to natural selection, because they occur
in different valleys on the same side of the mountain, where
food, climate, and enemies are the same ; and also, because
there is no greater difference in passing from the rainy to the
dry side of the mountains than in passing from one valley to

148 DARWINISM

another on the same side an equal distance apart. In a very
lengthy paper, presented to the Linnean Society last year, on
"Divergent Evolution through Cumulative Segregation," Mr.
Gulick endeavours to work out his views into a complete
theory, the main point of which may perhaps be indicated by
the following passage : " No two portions of a species possess
exactly the same average character, and the initial differences
are for ever reacting on the environment and on each other
in such a way as to ensure increasing divergence in each
successive generation as long as the individuals of the two
groups are kept from intercrossing."1

It need hardly be said that the views of Mr. Darwin and
myself are inconsistent with the notion that, if the environment
were absolutely similar for the two isolated portions of the
species, any such necessary and constant divergence would
take place. It is an error to assume that what seem to
us identical conditions are really identical to such small
and delicate organisms as these land molluscs, of whose
needs and difficulties at each successive stage of their existence,
from the freshly-laid egg up to the adult animal, we are so
profoundly ignorant. The exact proportions of the various
species of plants, the numbers of each kind of insect or of
bird, the peculiarities of more or less exposure to sunshine
or to wind at certain critical epochs, and other slight
differences which to us are absolutely immaterial and un-
recognisable, may be of the highest significance to these
humble creatures, and be quite sufficient to require some
slight adjustments of size, form, or colour, which natural
selection will bring about. All we know of the facts of
variation leads us to believe that, without this action of
natural selection, there would be produced over the whole area
a series of inconstant varieties mingled together, not a distinct
segregation of forms each confined to its own limited area.

Mr. Darwin has shown that, in the distribution and
modification of species, the biological is of more importance
than the physical environment, the struggle with other
organisms being often more severe than that with the forces
of nature. This is particularly evident in the case of plants,
many of Avhich, when protected from competition, thrive in a
1 Journal of the Linnean Society, Zoology, vol. xx. p. 215.

vi DIFFICULTIES AND OBJECTIONS 149

soil, climate, and atmosphere widely different from those of
their native habitat. Thus, many alpine plants only found
near perpetual snow thrive well in our gardens at the level of
the sea ; as do the tritomas from the sultry plains of South
Africa, the yuccas from the arid hills of Texas and Mexico, and
the fuchsias from the damp and dreary shores of the Straits of
Magellan. It has been well said that plants do not live
where they like, but where they can ; and the same remark will
apply to the animal world. Horses and cattle run wild and
thrive both in North and South America ; rabbits, once con-
fined to the south of Europe, have established themselves in
our own country and in Australia ; while the domestic fowl, a
native of tropical India, thrives well in every part of the
temperate zone.

If, then, we admit that when one portion of a species is
separated from the rest, there will necessarily be a slight
difference in the average characters of the two portions, it
does not follow that this difference has much if any effect
upon the characteristics that are developed by a long period
of isolation- In the first place, the difference itself will
necessarily be very slight unless there is an exceptional
amount of variability in the species ; and in the next place,
if the average characters of the species are the expression of
its exact adaptation to its whole environment, then, given
a precisely similar environment, and the isolated portion will
inevitably be brought back to the same average of characters.
But, as a matter of fact, it is impossible that the environment
of the isolated portion can be exactly like that of the bulk of
the species. It cannot be so physically, since no two separated
areas can be absolutely alike in climate and soil ; and even if
these are the same, the geographical features, size, contour, and
relation to winds, seas, and rivers, would certainly differ.
Biologically, the differences are sure to be considerable. The
isolated portion of a species will almost always be in a much
smaller area than that occupied by the species as a whole, hence
it is at once in a different position as regards its own kind.
The proportions of all the other species of animals and plants
are also sure to differ in the two areas, and some species will
almost always be absent in the smaller which are present in
the larger country. These differences will act and react on

150 DARWINISM chap.

the isolated portion of the species. The struggle for existence
will differ in its severity and in its incidence from that which
affects the bulk of the species. The absence of some one
insect or other creature inimical to the young animal or plant
may cause a vast difference in its conditions of existence, and
may necessitate a' modification of its external or internal
characters in quite a different direction from that which
happened to be present in the average of the individuals
which were first isolated.

On the whole, then, we conclude that, while isolation is an
important factor in effecting some modification of species, it is
so, not on account of any effect produced, or influence exerted
by isolation per se, but because it is always and necessarily
accompanied by a change of environment, both physical and
biological. Natural selection will then begin to act in
adapting the isolated portion to its new conditions, and will
do this the more quickly and the more effectually because of
the isolation. We have, however, seen reason to believe that
geographical or local isolation is by no means essential to the
differentiation of species, because the same result is brought
about by the incipient species acquiring different habits or
frequenting a different station ; and also by the fact that
different varieties of the same species are known to prefer to
pair with their like, and thus to bring about a physiological
isolation of the most effective kind. This part of the subject
will be again referred to when the very difficult problems
presented by hybridity are discussed.1

Cases in which Isolation is Ineffective.

One objection to the views of those who, like Mr. Gulick,
believe isolation itself to be a cause of modification of species
deserves attention, namely, the entire absence of change where,

1 In Mr. Gulick's last paper {Journal of Linn. Soc. Zool., vol. xx. pp. 189-
274) he discusses the various forms of isolation above referred to, under no
less than thirty-eight different divisions and subdivisions, with au elaborate
terminology, and he argues that these will frequently bring about divergent
evolution without any change in the environment or any action of natural
selection. The discussion of the problem here given will, I believe, sufficiently
expose the fallacy of his contention ; but his illustration of the varied and
often recondite modes by which practical isolation may be brought about,
may help to remove one of the popular difficulties in the way of the action
of natural selection in the origination of species.

vi DIFFICULTIES AND OBJECTIONS 151

if this wore a vera causa, Ave should expect to find it. In
Ireland we have an excellent test case, for we know that it
has been separated from Britain since the end of the glacial
epoch, certainly many thousand years. Yet hardly one of
its mammals, reptiles, or land molluscs has undergone the
slightest change, even although there is certainly a distinct
difference in the environment both inorganic and organic.
That changes have not occurred through natural selection, is
perhaps due to the less severe struggle for existence owing to
the smaller number of competing species ; but, if isolation
itself were an efficient cause, acting continuously and cumula-
tively, it is incredible that a decided change should not have
been produced in thousands of years. That no such change has
occurred in this, and many other cases of isolation, seems to
prove that it is not in itself a cause of modification.

There yet remain a number of difficulties and objections
relating to the question of hybridity, which are so important
as to require a separate chapter for their adequate discussion.

CHAPTER VII

ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES
AND THE USUAL STERILITY OF THEIR HYBRID OFFSPRING:

Statement of the problem — Extreme susceptibility of the reproductive
functions — Reciprocal crosses — Individual differences in respect to
cross - fertilisation — Dimorphism and trimorphism among plants —
Cases of the fertility of hybrids and of the infertility of mongrels
— The effects of close inter-breeding — Mr. Huth's objections — Fertile
hybrids among animals — Fertility of hybrids among plants — Cases of
sterility of mongrels — Parallelism between crossing and change of
conditions — Remarks on the facts of hybridity — Sterility due to
changed conditions and usually correlated with other characters —
Correlation of colour with constitutional peculiarities — The isolation
of varieties by selective association — The influence of natural selection
upon sterility and fertility — Physiological selection — Summary and
concluding remarks.

One of the greatest, or perhaps we may say the greatest, of
all the difficulties in the way of accepting the theory of
natural selection as a complete explanation of the origin of
species, has been the remarkable difference between varieties
and species in respect of fertility when crossed. Generally
speaking, it may be said that the varieties of any one species,
however different they may be in external appearance, are
perfectly fertile when crossed, and their mongrel offspring are
equally fertile when bred among themselves ; while distinct
species, on the other hand, however closely they may resemble
each other externally, are usually infertile when crossed, and
their hybrid offspring absolutely sterile. This used to be
considered a fixed law of nature, constituting the absolute test
and criterion of a species as distinct from a variety ; and so
long as it was believed that species were sejmrate creations, or

chap, vii ON THE INFERTILITY OF CROSSES 153

at all events had an origin quite distinct from that of varieties,
this law could have no exceptions, because, if any two species
had been found to be fertile when crossed and their hybrid
offspring to be also fertile, this fact would have been held to
prove them to be not species but varieties. On the other hand,
if two varieties had been found to be infertile, or their mongrel
offspring to be sterile, then it would have been said : These
are not varieties but true species. Thus the old theory led
to inevitable reasoning in a circle ; and what might be only a
rather common fact was elevated into a law which had no
exceptions.

The elaborate and careful examination of the whole subject
by Mr. Darwin, who has brought together a vast mass of
evidence from the experience of agriculturists and horti-
culturists, as well as from scientific experimenters, has demon-
strated that there is no such fixed law in nature as was
formerly supposed. He shows us that crosses between some
varieties are infertile or even sterile, while crosses between
some species are quite fertile ; and that there are besides a
number of curious phenomena connected with the subject
which render it impossible to believe that sterility is anything
more than an incidental property of species, due to the
extreme delicacy and susceptibility of the reproductive powers,
and dependent on physiological causes we have not yet been
able to trace. Nevertheless, the fact remains that most species
which have hitherto been crossed produce sterile hybrids, as
in the well-known case of the mule ; while almost all domestic
varieties, when crossed, produce offspring which are perfectly
fertile among themselves. I will now endeavour to give such
a sketch of the subject as may enable the reader to see some-
thing of the complexity of the problem, referring him to Mr.
Darwin's works for fuller details.

Extreme Susceptibility of the Reproductive Functions.

One of the most interesting facts, as showing how sus-
ceptible to changed conditions or to slight constitutional
changes are the reproductive powers of animals, is the very
general difficulty of getting those which are kept in confine-
ment to breed ; and this is frequently the only bar to
domesticating wild species. Thus, elephants, bears, foxes,

154 DARWINISM

and numbers of species of rodents, very rarely breed in
confinement ; while other species do so more or less freely.
Hawks, vultures, and owls hardly ever breed in confinement ;
neither did the falcons kept for hawking ever breed. Of the
numerous small seed -eating birds kept in aviaries, hardly
any breed, neither do parrots. Gallinaceous birds usually
breed freely in confinement, but some do not ; and even
the guans and curassows, kept tame by the South American
Indians, never breed. This shows that change of climate has
nothing to do with the phenomenon ; and, in fact, the same
species that refuse to breed in Europe do so, in almost every
case, when tamed or confined in their native countries. This
inability to reproduce is not due to ill- health, since many
of these creatures are perfectly vigorous and live very long.

With our true domestic animals, on the other hand,
fertility is perfect, and is very little affected by changed
conditions. Thus, we see the common fowl, a native of
tropical India, living and multiplying in almost every part of
the world ; and the same is the case with our cattle, sheep,
and goats, our dogs and horses, and especially with domestic
pigeons. It therefore seems probable, that this facility for
breeding under changed conditions was an original property
of the species which man has domesticated — a property
which, more than any other, enabled him to domesticate them.
Yet, even with these, there is evidence that great changes of
conditions affect the fertility. In the hot valleys of the
Andes sheep are less fertile ; while geese taken to the high
plateau of Bogota were at first almost sterile, but after some
generations recovered their fertility. These and many
other facts seem to show that, with the majority of animals,
even a slight change of conditions may produce infertility or
sterility ; and also that after a time, when the animal has
become thoroughly acclimatised, as it were, to the new
conditions, the infertility is in some cases diminished or
altogether ceases. It is stated by Bechstein that the canary
was long infertile, and it is only of late years that good
breeding birds have become common ; but in this case no
doubt selection has aided the change.

As showing that these phenomena depend on deep-seated
causes and are of a very general nature, it is interesting

vii ON THE INFERTILITY OF CROSSES 155

to note that they occur also in the vegetable kingdom.
Allowing for all the circumstances which are known to
prevent the production of seed, such as too great luxuriance
of foliage, too little or too much heat, or the absence of
insects to cross-fertilise the flowers, Mr. Darwin shows that
many species which grow and flower with us, apparently in
perfect health, yet never produce seed. Other plants are
affected by very slight changes of conditions, producing seed
freely in one soil and not in another, though apparently
growing equally well in both ; while, in some cases, a
difference of position even in the same garden produces a
similar result.1

Reciprocal Crosses.

Another indication of the extreme delicacy of the
adjustment between the sexes, which is necessary to produce
fertility, is afforded by the behaviour of many species and
varieties when reciprocally crossed. This will be best
illustrated by a few of the examples furnished us by Mr.
Darwin. The two distinct species of plants, Mirabilis jalapa
and M. longiflora, can be easily crossed, and will produce
healthy and fertile hybrids when the pollen of the latter is
applied to the stigma of the former plant. But the same
experimenter, Kolreuter, tried in vain, more than two hundred
times during eight years, to cross them by applying the pollen
of M. jalapa to the stigma of M. longiflora. In other cases two
plants are so closely allied that some botanists class them as
varieties (as with Matthiola annua and M. glabra), and yet
there is the same great difference in the result when they are
reciprocally crossed.

Individual Differences in respect to Cross-Fertilisation.

A still more remarkable illustration of the delicate
balance of organisation needful for reproduction, is afforded
by the individual differences of animals and plants, as regards
both their power of intercrossing with other individuals or
other species, and the fertility of the offspring thus produced.
Among domestic animals, Darwin states that it is by no means
rare to find certain males and females which will not breed

1 Darwin's Animals and Plants under Domestication, vol. ii. pp. 163-170.

156 DARWINISM

together, though both are known to be perfectly fertile with
other males and females. Cases of this kind have occurred
among horses, cattle, pigs, dogs, and pigeons ; and the
experiment has been tried so frequently that there can be no
doubt of the fact. Professor G. J. Romanes states that he
has a number of -additional cases of this individual incom-
patibility, or of absolute sterility, between two individuals,
each of which is perfectly fertile with other individuals.

During the numerous experiments that have been made
on the hybridisation of plants similar peculiarities have been
noticed, some individuals being capable, others incapable, of
being crossed with a distinct species. The same individual
peculiarities are found in varieties, species, and genera.
Kolreuter crossed five varieties of the common tobacco
(Nicotiana tabacum) with a distinct species, Nicotiana
glutinosa, and they all yielded very sterile hybrids ; but
those raised from one variety were less sterile, in all the
experiments, than the hybrids from the four other varieties.
Again, most of the species of the genus Nicotiana have been
crossed, and freely produce hybrids ; but one species, N.
acuminata, not particularly distinct from the others, could
neither fertilise, nor be fertilised by, any of the eight other
species experimented on. Among genera we find some —
such as Hippeastrum, Crinum, Calceolaria, Dianthus — almost
all the species of Avhich will fertilise other species and produce
hybrid offspring ; while other allied genera, as Zephyranthes
and Silene, notwithstanding the most persevering efforts, have
not produced a single hybrid even between the most closely
allied species.

Dimorphism and Trimorphism.

Peculiarities in the reproductive system affecting indi-
viduals of the same species reach their maximum in what are
called heterostylecl, or dimorphic and trimorphic flowers,
the phenomena presented by Avhich form one of the most
remarkable of Mr. Darwin's many discoveries. Our common
cowslip and primrose, as well as many other species of the
genus Primula, have two kinds of flowers in about equal
proportions. In one kind the stamens are short, being
situated about the middle of the tube of the corolla, while the

ON THE INFERTILITY OF CROSSES

157

style is long, the globular stigma appearing just in the centre
of the open flower. In the other kind the stamens are long,
appearing in the centre or throat of the flower, while the
style is short, the stigma being situated halfway down the
tube at the same level as the stamens in the other form.
These two forms have long been known to florists as the
"pin-eyed" and the "thrum -eyed," but they are called by
Darwin the long-styled and short-styled forms (see woodcut).

Long-styled form. Short-styled form.

Fig. 17. — Primula veris (Cowslip).

The meaning and use of these different forms was quite
unknown till Darwin discovered, first, that cowslips and
primroses are absolutely barren if insects are prevented from
visiting them, and then, what is still more extraordinary, that
each form is almost sterile when fertilised by its own pollen,
and comparatively infertile when crossed with any other
plant of its own form, but is perfectly fertile when the pollen
of a long -styled is carried to the stigma of a short- styled
plant, or vice versa. It will be seen, by the figures, that the
arrangement is such that a bee visiting the flowers will carry
the pollen from the long anthers of the short-styled form to
the stigma of the long-styled form, while it would never
reach the stigma of another plant of the short-styled form.

158 DARWINISM

But an insect visiting, first, a long-styled plant, would deposit
the pollen on the stigma of another plant of the same kind if
it were next visited ; and this is probably the reason why the
wild short-styled plants were found to be almost always most
jDroductive of seed, since they must be all fertilised by the
other form, whereas the long- styled plants might often be
fertilised by their own form. The whole arrangement,
however, ensures cross-fertilisation ; and this, as Mr. Darwin
has shown by copious experiments, adds both to the vigour
and fertility of almost all plants as well as animals.

Besides the primrose family, many other plants of several
distinct natural orders present similar phenomena, one or
two of the most curious of which must be referred to. The
beautiful crimson flax (Linum grandiflorum) has also two
forms, the styles only differing in length ; and in this case Mr.
Darwin found by numerous experiments, which have since
been repeated and confirmed by other observers, that each
form is absolutely sterile with pollen from another plant of
its own form, but abundantly fertile when crossed with any
plant of the other form. In this case the pollen of the two
forms cannot be distinguished under the microscope (whereas
that of the two forms of Primula differs in size and shape),
yet it has the remarkable property of being absolutely
powerless on the stigmas of half the plants of its own species.
The crosses between the opposite forms, which are fertile, are
termed by Mr. Darwin "legitimate," and those between
similar forms, which are sterile, "illegitimate"; and he
remarks that we have here, within the limits of the same
species, a degree of sterility which rarely occurs except
between plants or animals not only of different species but of
different genera.

But there is another set of plants, the trimorphic, in which
the styles and stamens have each three forms — long, medium,
and short, and in these it is possible to have eighteen different
crosses. By an elaborate series of experiments it was shown
that the six legitimate unions — that is, when a plant was
fertilised by pollen from stamens of length corresponding to
that of its style in the two other forms — were all abundantly
fertile ; while the twelve illegitimate unions, when a plant was
fertilised by pollen from stamens of a different length from its

vii ON THE INFERTILITY OF CROSSES 159

own style, in any of the three forms, were either comparatively
or wholly sterile. x

We have here a wonderful amount of constitutional
difference of the reproductive organs within a single species,
greater than usually occurs within the numerous distinct
species of a genus or group of genera ; and all this diversity
appears to have arisen for a purpose which has been obtained
by many other, and apparently simpler, changes of structure
or of function, in other plants. This seems to show us, in the
first place, that variations in the mutual relations of the repro-
ductive organs of different individuals must be as frequent as
structural variations have been shown to be ; and, also, that
sterility in itself can be no test of specific distinctness. But
this point will be better considered when we have further
illustrated and discussed the complex phenomena of hybridity.

Cases opthe Fertility of Hybrids, and of the Infertility of Mongrels.

I now propose to adduce a few cases in which it has been
proved, by experiment, that hybrids between two distinct
species are fertile inter se; and then to consider why it is that
such cases are so few in number.

The common domestic goose (Anser ferus) and the Chinese
goose (A. cygnoides) are very distinct species, so distinct that
some naturalists have placed them in different genera ; yet they
have bred together, and Mr. Eyton raised from a pair of these
hybrids a brood of eight. This fact was confirmed by Mr.
Darwin himself, who raised several fine birds from a pair of
hybrids which were sent him.2 In India, according to
Mr. Blyth and Captain Hutton, whole flocks of these hybrid
geese are kept in various parts of the country where neither
of the pure parent species exists, and as they are kept for
profit they must certainly be fully fertile.

Another equally striking case is that of the Indian humped
and the common cattle, species which differ osteologically, and
also in habits, form, voice, and constitution, so that they are
by no means closely allied ; yet Mr. Darwin assures us that he

1 For a full account of these interesting facts and of the various problems
to which they give rise, the reader must consult Darwin's volume on The
Different Forms of Flowers in Plants of the same Species, chaps, i.-iv.

2 See Nature, vol. xxi. p. 207.

160 DARWINISM

has received decisive evidence that the hybrids between these
are perfectly fertile inter se.

Dogs have been frequently crossed with wolves and with
jackals, and their hybrid offspring have been found to be fertile
inter se to the third or fourth generation, and then usually to
show some signs of sterility or of deterioration. The wolf
and dog may be originally the same species, but the jackal is
certainly distinct ; and the appearance of infertility or of weak-
ness is probably clue to the fact that, in almost all these experi-
ments, the offspring of a single pair — themselves usually from
the same litter — were bred in-and-in, and this alone sometimes
produces the most deleterious effects. Thus, Mr. Low in his
great work on the Domesticated Animals of Gimt Britain,
says : "If Ave shall breed a pair of dogs from the same litter,
and unite again the offspring of this pair, we shall produce at
once a feeble race of creatures ; and the process being repeated
for one or two generations more, the family will die out, or be
incapable of propagating their race. A gentleman of Scotland
made the experiment on a large scale with certain foxhounds,
and he found that the race actually became monstrous
and perished utterly." The same writer tells us that hogs
have been made the subject of similar experiments : "After a
few generations the victims manifest the change induced in the
system. They become of diminished size ; the bristles are
changed into hairs ; the limbs become feeble and short ; the
litters diminish in frequency, and in the number of the young
produced ; the mother becomes unable to nourish them, and,
if the experiment be carried as far as the case will allow, the
feeble, and frequently monstrous offspring, will be incapable of
being reared up, and the miserable race will utterly perish."1

These precise statements, by one of the greatest authorities
on our domesticated animals, are sufficient to show that the
fact of infertility or degeneracy appearing in the offspring of
hybrids after a few generations need not be imputed to the
fact of the first parents being distinct species, since exactly the
same phenomena appear when individuals of the same species
are bred under similar adverse conditions. But in almost all
the experiments that have hitherto been made in crossing
distinct species, no care has been taken to avoid close inter-

1 Low's Domesticated Animals of Great Britain, Introduction, p. lxiv.

vii ON THE INFERTILITY OE CROSSES 161

breeding by securing several hybrids from quite distinct
stocks to start with, and by having two or more sets of experi-
ments carried on at once, so that crosses between the hybrids
produced may be occasionally made. Till this is done no
experiments, such as those hitherto tried, can be held to prove
that hybrids are in all cases infertile inter se.

It has, however, been denied by Mr. A. H. Huth, in his
interesting work on The Marriage of Near Kin, that any
amount of breeding in-and-in is in itself hurtful ; and he quotes
the evidence of numerous breeders whose choicest stocks have
always been so bred, as well as cases like the Porto Santo
rabbits, the goats of Juan Fernandez, and other cases in which
animals allowed to run wild have increased prodigiously and
continued in perfect health and vigour, although all derived
from a single pair. But in all these cases there has been
rigid selection by which the weak or the infertile have been
eliminated, and with such selection there is no doubt that the
ill effects of close interbreeding can be prevented for a long
time ; but this by no means proves that no ill effects are pro-
duced. Mr. Huth himself quotes M. Allie, M. Aube, Stephens,
Giblett, Sir John Sebright, Youatt, Druce, Lord Weston, and
other eminent breeders, as finding from experience that close
interbreeding does produce bad effects ; and it cannot be
supposed that there would be such a consensus of opinion
on this point if the evil were altogether imaginary. Mr.
Huth argues, that the evil results which do occur do not
depend on the close interbreeding itself, but on the tendency
it has to perpetuate any constitutional weakness or other
hereditary taints ; and he attempts to prove this by the argu-
ment that " if crosses act by virtue of being a cross, and not
by virtue of removing an hereditary taint, then the greater the
difference between the two animals crossed the more beneficial
will that act be." He then shows that, the wider the difference
the less is the benefit, and concludes that a cross, as such, has
no beneficial effect. A parallel argument would be, that change
of air, as from inland to the sea-coast, or from a low to an
elevated site, is not beneficial in itself, because, if so, a change
to the tropics or to the polar regions should be more beneficial.
In both these cases it may well be that no benefit would
accrue to a person in perfect health ; but then there is no

M

162 DARWINISM

such thing as " perfect health " in man, and probably no such
thing as absolute freedom from constitutional taint in animals.
The experiments of Mr. Darwin, showing the great and
immediate good effects of a cross between distinct strains in
plants, cannot be explained away ; neither can the innumerable
arrangements to secure cross-fertilisation by insects, the real
use and purport of which will be discussed in our eleventh
chapter. On the whole, then, the evidence at our command
proves that, whatever may be its ultimate cause, close inter-
breeding does usually produce bad results ; and it is only by
the most rigid selection, whether natural or artificial, that
the danger can be altogether obviated.

Fertile Hybrids among Animals.

One or two more cases of fertile hybrids may be given
before Ave pass on to the corresponding experiments in plants.
Professor Alfred Newton received from a friend a pair of
hybrid ducks, bred from a common duck (Anas boschas), and a
pintail (Dafila acuta). From these he obtained four ducklings,
but these latter, when grown up, proved infertile, and did not
breed again. In this case we have the results of close inter-
breeding, with too great a difference between the original
species, combining to produce infertility, yet the fact of a
hybrid from such a pair producing healthy offspring is itself
noteworthy.

Still more extraordinary is the following statement of Mr.
Low : "It has been long known to shepherds, though ques-
tioned by naturalists, that the progeny of the cross between the
sheep and goat is fertile. Breeds of this mixed race are
numerous in the north of Europe." 1 Nothing appears to be
known of such hybrids either in Scandinavia or in Italy ; but
Professor Giglioli of Florence has kindly given me some useful
references to works in which they are described. The following
extract from his letter is very interesting : " I need not tell
you that there being such hybrids is now generally accepted as
a fact. Buffon (Supplements, torn. iii. p. 7, 1756) obtained one
such hybrid in 1751 and eight in 1752. Sanson (La Culture,
vol. vi. p. 372, 1865) mentions a case observed in the Vosges,
France. Geoff. St. Hilaire (Hist. Nat. GSn. des reg. org., vol. iii. p.
1 Low's Domesticated Animals, p. 28.

vii ON THE INFERTILITY OF CROSSES 163

163) was the first to mention, I believe, that in different parts
of South America the ram is more usually crossed with the
she-goat than the sheep with the he-goat. The well-known
' pellones ' of Chile are produced by the second and third
generation of such hybrids (Gay, 'Hist, de Chile,7 vol. i. p. 466,
Agriculture, 1862). Hybrids bred from goat and sheep are
called 'chabin' in French, and 'cabruno' in Spanish. In
Chile such hybrids are called ' carneros lanudos ' ; their breed-
ing inter se appears to be not always successful, and often the
original cross has to be recommenced to obtain the proportion of
three-eighths of he-goat and five-eighths of sheep, or of three-
eighths of ram and five-eighths of she-goat ; such being the
reputed best hybrids."

With these numerous facts recorded by competent observers
we can hardly doubt that races of hybrids between these very
distinct species have been produced, and that such hybrids are
fairly fertile inter se; and the analogous facts already given lead
us to believe that whatever amount of infertility may at first
exist could be eliminated by careful selection, if the crossed
races were bred in large numbers and over a considerable area
of country. This case is especially valuable, as showing how
careful we should be in assuming the infertility of hybrids
when experiments have been made with the progeny of a single
pair, and have been continued only for one or two generations.

Among insects one case only appears to have been recorded.
The hybrids of two moths (Bombyx cynthia and B. arrindia)
were proved in Paris, according to M. Quatrefages, to be fertile
inter se for eight generations.

Fertility of Hybrids among Plants.

Among plants the cases of fertile hybrids are more numerous,
owing, in part, to the large scale on which they are grown by
gardeners and nurserymen, and to the greater facility with
which experiments can be made. Darwin tells us that Kolreuter
found ten cases in which two plants considered by botanists
to be distinct species were quite fertile together, and he there-
fore ranked them all as varieties of each other. In some
cases these were grown for six to ten successive generations, but
after a time the fertility decreased, as we saw to be the case in

164 DARWINISM

animals, and presumably from the same cause, too close inter-
breeding.

Dean Herbert, who carried on experiments with great care
and skill for many years, found numerous cases of hybrids
which were perfectly fertile inter se. Crinum capense, fertilised
by three other species — C. pedunculatum, C. canaliculatum, or
C. defixum — all very distinct from it, produced perfectly
fertile hybrids ; while other species less different in appearance
were quite sterile with the same C. capense.

All the species of the genus Hippeastrum produce hybrid
offspring which are invariably fertile. Lobelia syphylitica and
L. fulgens, two very distinct species, have produced a hybrid
which has been named Lobelia speciosa, and which reproduces
itself abundantly. Many of the beautiful pelargoniums of
our greenhouses are hybrids, such as P. ignescens from a cross
between P. citrinodorum and P. fulgidum, which is quite
fertile, and has become the parent of innumerable varieties of
beautiful plants. All the varied species of Calceolaria, how-
ever different in appearance, intermix with the greatest readi-
ness, and the hybrids are all more or less fertile. But the
most remarkable case is that of two species of Petunia, of which
Dean Herbert says : " It is very remarkable that, although
there is a great difference in the form of the flower, especially
of the tube, of P. nyctanigenaaflora and P. phoenicea the
mules between them are not only fertile, but I have found
them seed much more freely with me than either parent.
.... From a pod of the above-mentioned mule, to which
no pollen but its own had access, I had a large batch of seed-
lings in which there was no variability or difference from
itself ; and it is evident that the mule planted by itself, in a
congenial climate, would reproduce itself as a species ; at least
as much deserving to be so considered as the various Calceo-
larias of different districts of South America."1

Darwin was informed by Mr. C. Noble that he raises stocks
for grafting from a hybrid between Rhododendron ponticum
and E. catawbiense, and that this hybrid seeds as freely as it
is possible to imagine. He adds that horticulturists raise
large beds of the same hybrid, and such alone are fairly
treated ; for, by insect agency, the several individuals arc freely

1 Amaryllidacece, by the Hon. and Rev. William Herbert, p. 379.

vii ON THE INFERTILITY OF CROSSES 165

crossed with each other, and the injurious influence of close
interbreeding is thus prevented. Had hybrids, when fairly
treated, always gone on decreasing in fertility in each suc-
cessive generation, as Gartner believed to be the case, the fact
would have been notorious to nurserymen.1

Cases of Sterility of Mongrels.

The reverse phenomenon to the fertility of hybrids, the
sterility of mongrels or of the crosses between varieties of the
same species, is a comparatively rare one, yet some undoubted
cases have occurred. Gartner, who believed in the absolute
distinctness of species and varieties, had two varieties of
maize — one dwarf with yellow seeds, the other taller with red
seeds ; yet they never naturally crossed, and, when fertilised
artificially, only a single head produced any seeds, and this one
only five- grains. Yet these few seeds were fertile ; so that in
this case the first cross was almost sterile, though the hybrid
when at length produced was fertile. In like manner, dis-
similarly coloured varieties of Verbascum or mullein have been
found by two distinct observers to be comparatively infertile.
The two pimpernels (Anagallis arvensis and A. coerulea), classed
by most botanists as varieties of one species, have been found,
after repeated trials, to be perfectly sterile when crossed.

No cases of this kind are recorded among animals ; but
this is not to be wondered at, when we consider how very few
experiments have been made with natural varieties; while
there is good reason for believing that domestic varieties are
exceptionally fertile, partly because one of the conditions of
domestication was fertility under changed conditions, and also
because long continued domestication is believed to have the
effect of increasing fertility and eliminating whatever sterility
may exist. This is shown by the fact that, in many cases,
domestic animals are descended from two or more distinct
species. This is almost certainly the case with the dog, and
probably with the hog, the ox, and the sheep ; yet the various
breeds are now all perfectly fertile, although we have every
reason to suppose that there would be some degree of infer-
tility if the several aboriginal species were crossed together
for the first time.

1 Origin of Species, p. 239.

166 DARWINISM

Parallelism between Crossing and Change of Conditions.

In the whole series of these phenomena, from the beneficial
effects of the crossing of different stocks and the evil effects of
close interbreeding, up to the partial or complete sterility
induced by crosses between species belonging to different
genera, we have, as Mr. Darwin points out, a curious parallelism
with the effects produced by change of physical conditions.
It is well known that slight changes in the conditions of life
are beneficial to all living things. Plants, if constantly grown
in one soil and locality from their own seeds, are greatly
benefited by the importation of seed from some other locality.
The same thing happens with animals ; and the benefit we our-
selves experience from " change of air " is an illustration of
the same phenomenon. But the amount of the change which
is beneficial has its limits, and then a greater amount is
injurious. A change to a climate a few degrees warmer or
colder may be good, while a change to the tropics or to the
arctic regions might be injurious.

Thus we see that, both slight changes of conditions and
a slight amount of crossing, are beneficial ; while extreme
changes, and crosses between individuals too far removed in
structure or constitution, are injurious. And there is not
only a parallelism but an actual connection between the two
classes of facts, for, as we have already shown, many species
of animals and plants are rendered infertile, or altogether
sterile, by the change from their natural conditions which
occurs in confinement or in cultivation ; while, on the other
hand, the increased vigour or fertility which is invariably pro-
duced by a judicious cross may be also effected by a judicious
change of climate and surroundings. We shall see in a subse-
quent chapter, that this interchangeability of the beneficial effects
of crossing and of new conditions, serves to explain some very
puzzling phenomena in the forms and economy of flowers.

Remarks on the Facts of Hybridity.

The facts that have now been adduced, though not very
numerous, are sufficiently conclusive to prove that the old
belief, of the universal sterility of hybrids and fertility of
mongrels, is incorrect. The doctrine that such a universal

vii ON THE INFERTILITY OF CROSSES 167

law existed was never more than a plausible generalisa-
tion, founded on a few inconclusive facts derived from
domesticated animals and cultivated plants. The facts were,
and still are, inconclusive for several reasons. They are
founded, primarily, on what occurs among animals in
domestication ; and it has been shown that domestication
both tends to increase fertility, and was itself rendered
possible by the fertility of those particular species being little
affected by changed conditions. The exceptional fertility of
all the varieties of domesticated animals does not prove that
a similar fertility exists among natural varieties. In the next
place, the generalisation is founded on too remote crosses, as in
the case of the horse and the ass, the two most distinct and
widely separated species of the genus Equus, so distinct indeed
that they have been held by some naturalists to form distinct
genera. Crosses between the two species of zebra, or even
between the zebra and the quagga, or the quagga and the ass,
might have led to a very different result. Again, in pre-
Darwinian times it was so universally the practice to argue in
a circle, and declare that the fertility of the offspring of a
cross proved the identity of species of the parents, that experi-
ments in hybridity were usually made between very remote
species and even between species of different genera, to avoid
the possibility of the reply : " They are both really the same
species ;" and the sterility of the hybrid offspring of such
remote crosses of course served to strengthen the popular
belief.

Now that we have arrived at a different standpoint, and
look upon a species, not as a distinct entity due to special
creation, but as an assemblage of individuals which have become
somewhat modified in structure, form, and constitution so as
to adapt them to slightly different conditions of life ; which
can be differentiated from other allied assemblages ; which
reproduce their like, and which usually breed together — we
require a fresh set of experiments calculated to determine the
matter of fact, — whether such species crossed with their near
allies do always produce offspring which are more or less
sterile inter se. Ample materials for such experiments exist,
in the numerous "representative species" inhabiting distinct
areas on a continent or different islands of a group ; or even

168 DARWINISM

in those found in the same area but frequenting somewhat
different stations.

To carry out these experiments with any satisfactory result,
it will be necessary to avoid the evil effects of confinement
and of too close interbreeding. If birds are experimented
with, they should .be allowed as much liberty as possible, a
plot of ground with trees and bushes being enclosed with
wire netting overhead so as to form a large open aviary.
The species experimented with should be obtained in con-
siderable numbers, and by two separate persons, each making
the opposite reciprocal cross, as explained at p. 155. In the
second generation these two stocks might be themselves crossed
to prevent the evil effects of too close interbreeding. By such
experiments, carefully carried out with different groups of
animals and plants, we should obtain a body of facts of a
character now sadly wanting, and without which it is hopeless
to expect to arrive at a complete solution of this difficult
problem. There are, however, some other aspects of the
question that need to be considered, and some theoretical
views which require to be carefully examined, having done
which we shall be in a condition to state the general con-
clusions to which the facts and reasonings at our command
seem to point.

Sterility due to changed Conditions and usually correlated with
other Characters, especially with Colour.

The evidence already adduced as to the extreme suscep-
tibility of the reproductive system, and the curious irregu-
larity with which infertility or sterility appears in the crosses
between some varieties or species while quite absent in those
between others, seem to indicate that sterility is a charac-
teristic which has a constant tendency to appear, either by
itself or in correlation with other characters. It is known
to be especially liable to occur under changed conditions of
life ; and, as such change is usually the starting-point and
cause of the development of new species, we have already
found a reason why it should so often appear when species
become fully differentiated.

In almost all the cases of infertility or sterility between
varieties or species, we have some external differences with

vit ON THE INFERTILITY OF CROSSES 169

which it is coiTelated ; and though these differences are
sometimes slight, and the amount of the infertility is not
always, or even usually, proportionate to the external dif-
ference between the two forms crossed, we must believe that
there is some connection between the two classes of facts.
This is especially the case as regards colour ; and Mr. Darwin
has collected a body of facts which go far to prove that
colour, instead of being an altogether trifling and un-
important character, as was supposed by the older natural-
ists, is really one of great significance, since it is un-
doubtedly often correlated with important constitutional
differences. Now colour is one of the characters that most
usually distinguishes closely allied species ; and when we
hear that the most closely allied species of plants are
infertile together, while those more remote are fertile, the
meaning usually is that the former differ chiefly in the colour
of their flowers, while the latter differ in the form of the
flowers or foliage, in habit, or in other structural characters.

It is therefore a most curious and suggestive fact, that in
all the recorded cases, in which a decided infertility occurs
between varieties of the same species, those varieties are
distinguished by a difference of colour. The infertile
varieties of Verbascum were white and yellow flowered
respectively ; the infertile varieties of maize were red and
yellow seeded ; while the infertile pimpernels were the red
and the blue flowered varieties. So, the differently coloured
varieties of hollyhocks, though grown close together, each
reproduce their own colour from seed, showing that they are
not capable of freely intercrossing. Yet Mr. Darwin assures
us that the agency of bees is necessary to carry the pollen
from one plant to another, because in each flower the pollen
is shed before the stigma is ready to receive it. We have
here, therefore, either almost complete sterility between
varieties of different colours, or a prepotent effect of pollen
from a flower of the same colour, bringing about the same
result.

Similar phenomena have not been recorded among
animals ; but this is not to be wondered at when we consider
that most of our pure and valued domestic breeds are
characterised by definite colours which constitute one of their

170 DARWINISM

distinctive marks, and they are, therefore, seldom crossed with
these of another colour ; and even when they are so crossed, no
notice would be taken of any slight diminution of fertility, since
this is liable to occur from many causes. We have also reason
to believe that fertility has been increased by long domestica-
tion, in addition to the fact of the original stocks being
exceptionally fertile ; and no experiments have been made on
the differently coloured varieties of wild animals. There are,
however, a number of very curious facts showing that colour
in animals, as in plants, is often correlated with constitutional
differences of a remarkable kind, and as these have a close
relation to the subject we are discussing, a brief summary of
them will be here given.

Correlation of Colour with Constitutional Peculiarities.

The correlation of a white colour and blue eyes in male
cats with deafness, and of the tortoise-shell marking with the
female sex of the same animal, are two well-known but most
extraordinary cases. Equally remarkable is the fact, com-
municated to Darwin by Mr. Tegetmeier, that white, yellow,
pale blue, or dun pigeons, of all breeds, have the young birds
born naked, while in all other colours they are well covered
with down. Here we have a case in which colour seems of
more physiological importance than all the varied structural
differences between the varieties and breeds of pigeons.
In Virginia there is a plant called the paint-root (Lachnanthes
tinctoria), which, when eaten by pigs, colours their bones
pink, and causes the hoofs of all but the black varieties to
drop off; so that black pigs only can be kept in the district.1
Buckwheat in flower is also said to be injurious to white
pigs but not to black. In the Tarentino, black sheep
are not injured by eating the Hypericum crispum — a species
of St. John's-wort — which kills white sheep. White terriers
suffer most from distemper ; white chickens from the gapes.
White-haired horses or cattle are subject to cutaneous
diseases from which the dark coloured are free ; while, both in
Thuringia and the West Indies, it has been noticed that white
or pale coloured cattle are much more troubled by flies than are
those which are brown or black. The same law even extends

1 Origin of Species, sixth edition, p. 9.

vii ON" THE INFERTILITY OF CROSSES 171

to insects, for it is found that silkworms which produce white
cocoons resist the fungus disease much better than do those
which produce yellow cocoons.1 Among plants, we have in
North America green and yellow-fruited plums not affected by
a disease that attacked the purple-fruited varieties. Yellow-
fleshed peaches suffer more from disease than white-fleshed
kinds. In Mauritius, white sugar-canes were attacked by a
disease from which the red canes were free. White onions
and verbenas are most liable to mildew ; and red-flowered
hyacinths were more injured by the cold during a severe
winter in Holland than any other kinds.2

These curious and inexplicable correlations of colour with
constitutional peculiarities, both in animals and plants, render
it probable that the correlation of colour with infertility,
which has been detected in several cases in plants, may also
extend .to animals in a state of nature ; and if so, the fact
is of the highest importance as throwing light on the origin
of the infertility of many allied species. This will be better
understood after considering the facts which will be now
described.

The Isolation of Varieties by Selective Association.

In the last chapter I have shown that the importance of
geographical isolation for the formation of new species by
natural selection has been greatly exaggerated, because the

1 In the Medico- Chirurgical Transactions, vol. liii. (1870), Dr. Ogle has
adduced some curious physiological facts bearing on the presence or absence
of white colours in the higher animals. He states that a dark pigment in the
olfactory region of the nostrils is essential to perfect smell, and that this
pigment is rarely deficient except when the whole animal is pure white, and
the creature is then almost without smell or taste. He observes that there is
no proof that, in any of the cases given above, the black animals actually eat
the poisonous root or plant ; and that the facts are readily understood if the
senses of smell and taste are dependent on a pigment which is absent in the
white animals, who therefore eat what those gifted with normal senses avoid.
This explanation however hardly seems to cover the facts. "We cannot sup-
pose that almost all the sheep in the world (which are mostly white) are
without smell or taste. The cutaneous disease on the white patches of hair
on horses, the special liability of white terriers to distemper, of white chickens
to the gapes, and of silkworms which produce yellow silk to the fungus, are
not explained by it. The analogous facts in plants also indicate a real con-
stitutional relation with colour, not an affection of the sense of smell and
taste only.

2 For all these facts, see Animals and Plants under Domestication, vol. ii.
pp. 335-338.

172 DARWINISM

very change of conditions, which is the initial power in
starting such new forms, leads also to a local or stational
segregation of the forms acted upon. But there is also a very-
powerful cause of isolation in the mental nature — the likes
and dislikes — of animals ; and to this is probably due the fact
of the comparative rarity of hybrids in a state of nature.
The differently coloured herds of cattle in the Falkland Islands,
each of which keeps separate, have been already mentioned ;
and it may be added, that the mouse-coloured variety seem
to have already developed a physiological peculiarity in breed-
ing a month earlier than the others. Similar facts occur,
however, among our domestic animals and are well known to
breeders. Professor Low, one of the greatest authorities on
our domesticated animals, says : " The female of the dog, when
not under restraint, makes selection of her mate, the mastiff
selecting the mastiff, the terrier the terrier, and so on." And
again : " The Merino sheep and Heath sheep of Scotland, if
two flocks are mixed together, each will breed with its own
variety." Mr. Darwin has collected many facts illustrating
this point. One of the chief pigeon-fanciers in England
informed him that, if free to choose, each breed would prefer
pairing with its own kind. Among the wild horses in Para-
guay those of the same colour and size associate together;
while in Circassia there are three races of horses which have
received special names, and which, when living a free life,
almost always refuse to mingle and cross, and will even
attack one another. On one of the Faroe Islands, not more
than half a mile in diameter, the half-wild native black sheep
do not readily mix with imported white sheep. In the
Forest of Dean, and in the New Forest, the dark and pale
coloured herds of fallow deer have never been known to
mingle ; and even the curious Ancon sheep of quite modern
origin have been observed to keep together, separating them-
selves from the rest of the flock when put into enclosures
with other sheep. The same rule applies to birds, for Darwin
was informed by the Rev. W, D. Fox that his flocks of white
and Chinese geese kept distinct.1

This constant preference of animals for their like, even in the
case of slightly different varietiesof the same species, is evidently
1 Animals and Plants under Domestication, vol. ii. pp. 102, 103.

vi i ON THE INFERTILITY OF CROSSES 17;;

a fact of great importance in considering the origin of species
by natural selection, since it shows ns that, so soon as a slight
differentiation of form or colour has been effected, isolation
will at once arise by the selective association of the animals
themselves; and thus the great stumbling-block of "the
swamping effects of intercrossing," which has been so pro-
minently brought forward by many naturalists, will be com-
pletely obviated.

If now we combine with this fact the correlation of colour
with important constitutional peculiarities, and, in some cases,
with infertility ; and consider, further, the curious parallelism
that has been shown to exist between the effects of changed
conditions and the intercrossing of varieties in producing
either an increase or a decrease of fertility, we shall have
obtained, at all events, a starting-point for the production of
that infertility which is so characteristic a feature of distinct
species when intercrossed. All we need, noAv, is some means
of increasing or accumulating this initial tendency ; and to a
discussion of this problem we will therefore address ourselves.

The Influence of Natural Selection upon Sterility and Fertility.
It will occur to many persons that, as the infertility or
sterility of incipient species would be useful to them when
occupying the same or adjacent areas, by neutralising the
effects of intercrossing, this infertility might have been in-
creased by the action of natural selection ; and this will be
thought the more probable if we admit, as we have seen
reason to do, that variations in fertility occur, perhaps as
frequently as other variations. Mr. Darwin tells us that, at
one time, this appeared to him probable, but he found the
problem to be one of extreme complexity ; and he was also
influenced against the view by many considerations which
seemed to render such an origin of the sterility or infertility
of species when intercrossed very improbable. The fact that
species which occupy distinct areas, and which nowhere come
in contact with each other, are often sterile when crossed, is one
of the difficulties ; but this may perhaps be overcome by the
consideration that, though now isolated, they may, and often
must, have been in contact at their origination. More
important is the objection that natural selection could not

174 DARWINISM chap.

possibly have produced the difference that often occurs
between reciprocal crosses, one of these being sometimes
fertile, while the other is sterile. The extremely different
amounts of infertility or sterility between different species
of the same genus, the infertility often bearing no proportion
to the difference -between the species crossed, is also an
important objection. But none of these objections would
have much weight if it could be clearly shown that natural
selection is able to increase the infertility variations of in-
cipient species, as it is certainly able to increase and develop
all useful variations of form, structure, instincts, or habits.
Ample causes of infertility have been shown to exist, in the
nature of the organism and the laws of correlation; the
agency of natural selection is only needed to accumulate
the effects produced by these causes, and to render their final
results more uniform and more in accordance with the facts
that exist.

About twenty years ago I had much correspondence and
discussion with Mr. Darwin on this question. I then believed
that I was able to demonstrate the action of natural selection in
accumulating infertility ; but I could not convince him, owing
to the extreme complexity of the process under the conditions
which he thought most probable. I have recently returned
to the question ; and, with the fuller knowledge of the facts of
variation we now possess, I think it may be shown that
natural selection is, in some probable cases at all events, able
to accumulate variations in infertility between incipient species.

The simplest case to consider, will be that in which two
forms or varieties of a species, occupying an extensive area, are
in process of adaptation to somewhat different modes of life
within the same area. If these two forms freely intercross
with each other, and produce mongrel offspring which are
quite fertile inter se, then the further differentiation of the
forms into two distinct species will be retarded, or perhaps
entirely prevented ; for the offspring of the crossed unions
will be, perhaps, more vigorous on account of the cross,
although less perfectly adapted to the conditions of existence
than either of the pure breeds ; and this would certainly estab-
lish a powerful antagonistic influence to the further differentia-
tion of the two forms.

vii ON THE INFERTILITY OF CROSSES 175

Now, let us suppose that a partial sterility of the hybrids
between the two forms arises, in correlation with the different
modes of life and the slight external or internal peculiarities
that exist between them, both of which we have seen to be
real causes of infertility. The result will be that, even if the
hybrids between the two forms are still freely produced, these
hybrids will not themselves increase so rapidly as the two
pure forms; and as these latter are, by the terms of the
problem, better suited to their conditions of life than are
the hybrids between them, they will not only increase more
rapidly, but will also tend to supplant the hybrids altogether
whenever the struggle for existence becomes exceptionally
severe. Thus, the more complete the sterility of the hybrids
the more rapidly will they die out and leave the two parent
forms pure. Hence it will follow that, if there is greater
infertility between the two forms in one part of the area than
the other, these forms will be kept more pure wherever
this greater infertility prevails, will therefore have an
advantage at each recurring period of severe struggle for
existence, and will thus ultimately supplant the less infertile
or completely fertile forms that may exist in other portions
of the area. It thus appears that, in such a case as here
supposed, natural selection would preserve those portions of
the two breeds which were most infertile with each other, or
Avhose hybrid offspring were most infertile ; and would,
therefore, if variations in fertility continued to arise, tend to
increase that infertility. It must particularly be noted that
this effect would result, not by the preservation of the
infertile variations on account of their infertility, but by the
inferiority of the hybrid offspring, both as being fewer in
numbers, less able to continue their race, and less adapted to
the conditions of existence than either of the pure forms. It
is this inferiority of the hybrid offspring that is the essential
point ; and as the number of these hybrids will be per-
manently less where the infertility is greatest, therefore those
portions of the two forms in which infertility is greatest will
have the advantage, and will ultimately survive in the struggle
for existence.

The differentiation of the two forms into distinct sjjecies,
with the increase of infertility between them, would be

176 • DARWINISM

greatly assisted by two other important factors in the
problem. It has already been shown that, with each
modification of form and habits, and especially with modifica-
tions of colour, there arises a disinclination of the two forms
to pair together; and this would produce an amount of
isolation which would greatly assist the specialisation of the
forms in adaptation to their different conditions of life.
Again, evidence has been adduced that change of conditions
or of mode of life is a potent cause of disturbance of the
reproductive system, and, consequently, of infertility. We
may therefore assume that, as the two forms adopted more
and more different modes of life, and perhaps acquired also
decided peculiarities of form and coloration, the infertility
between them would increase or become more general ; and as
we have seen that every such increase of infertility would
give that portion of the species in which it arose an advantage
over the remaining portions in which the two varieties were
more fertile together, all this induced infertility would main-
tain itself, and still further increase the general infertility be-
tween the two forms of the species.

It follows, then, that specialisation to separate conditions
of life, differentiation of external characters, disinclination to
cross-unions, and the infertility of the hybrid produce of these
unions, would all proceed pari passu, and would ultimately
lead to the production of two distinct forms having all the
characteristics, physiological as well as structural, of true
species.

In the case now discussed it has been supposed, that some
amount of general infertility might arise in correlation with
the different modes of life of two varieties or incipient
species. A considerable body of facts already adduced
renders it probable that this is the mode in which any
widespread infertility Avould arise ; and, if so, it has been
shown that, by the influence of natural selection and the
known laws which affect varieties, the infertility would be
gradually increased. But, if we suppose the infertility to
arise sporadically within the two forms, and to affect only a
small proportion of the individuals in any area, it will be
difficult, if not impossible, to show that such infertility would
have any tendency to increase, or would produce any but a

vii ON THE INFERTILITY OF CROSSES 177

prejudicial effect. If, for example, five per cent of each
form thus varied so as to be infertile with the other form,
the result would be hardly perceptible, because the individuals
which formed cross-unions and produced hybrids would con-
stitute a very small portion of the whole species ; and the
hybrid offspring, being at a disadvantage in the struggle for
existence and being themselves infertile, would soon die out,
while the much more numerous fertile portion of the two
forms Avoulcl increase rapidly, and furnish a sufficient number
of pure-bred offspring of each form to take the place of the
somewhat inferior hybrids between them whenever the
struggle for existence became severe. We must suppose that
the normal fertile forms would transmit their fertility to their
progeny, and the few infertile forms their infertility ; but
the latter would necessarily lose half their proper increase
by the sterility of their hybrid offspring whenever they
crossed with the other form, and when they bred with their
own form the tendency to sterility would die out except in
the very minute proportion of the five per cent (one-twentieth)
that chance would lead to pair together. Under these
circumstances the incipient sterility between the two forms
would rapidly be eliminated, and could never rise much above
the numbers which were produced by sporadic variation each
year.

It was, probably, by a consideration of some such case as
this that Mr. Darwin came to the conclusion that infertility
arising between incipient species could not be increased by
natural selection ; and this is the more likely, as he was
always disposed to minimise both the frequency and the
amount even of structural variations.

We have yet to notice another mode of action of natural
selection in favouring and perpetuating any infertility that
may arise between two incipient species. If several distinct
species are undergoing modification at the same time and in
the same area, to adapt them to some new conditions that
have arisen there, then any species in which the structural or
colour differences that have arisen between it and its varieties
or close allies were correlated with infertility of the crosses
between them, would have an advantage over the corre-
sponding varieties of other species in which there was no such

N

178 DARWINISM

physiological peculiarity. Thus, incipient species which were
infertile together would have an advantage over other
incipient species which were fertile, and, whenever the
struggle for existence became severe, would prevail over them
and take their place. Such infertility, being correlated with
constitutional or 'structural differences, would probably, as
already suggested, go on increasing as these differences
increased ; and thus, by the time the new species became
fully differentiated from its parent form (or brother variety)
the infertility might have become as well marked as we
usually find it to be between distinct species.

This discussion has led us to some conclusions of the greatest
importance as bearing on the difficult problem of the cause of
the sterility of the hybrids between distinct species. Accept-
ing, as highly probable, the fact of variations in fertility
occurring in correlation with variations in habits, colour, or
structure, we see, that so long as such variations occurred only
sporadically, and affected but a small proportion of the in-
dividuals in any area, the infertility could not be increased by
natural selection, but would tend to die out almost as fast as
it was produced. If, however, it was so closely correlated
with physical variations or diverse modes of life as to
affect, even in a small degree, a considerable proportion of
the individuals of the two forms in definite areas, it would
be preserved by natural selection, and the portion of the
varying species thus affected would increase at the expense of
those portions which were more fertile when crossed. Each
further variation towards infertility between the two forms
would be again preserved, and thus the incipient infertility
of the hybrid offspring might be increased till it became so
great as almost to amount to sterility. Yet further, we have
seen that if several competing species in the same area were
being simultaneously modified, those between Avhose varieties
infertility arose would have an advantage over those whose
varieties remained fertile inter se, and would ultimately sup-
plant them.

The preceding argument, it will be seen, depends entirely
upon the assumption that some amount of infertility char-
acterises the distinct varieties which are in process of
differentiation into species ; and it may be objected that of

vii ON THE INFERTILITY OF CROSSES 179

such infertility there is no proof. This is admitted ; but it is
urged that facts have been adduced which render such
infertility probable, at least in some cases, and this is all
that is required. It is by no means necessary that all varieties
should exhibit incipient infertility, but only some varieties ;
for we know that, of the innumerable varieties that occur
but few become developed into distinct species, and it may be
that the absence of infertility, to obviate the effects of inter-
crossing, is one of the usual causes of their failure. All I
have attempted to show is, that when incipient infertility does
occur in correlation with other varietal differences, that in-
fertility can be, and in fact must be, increased by natural
selection ; and this, it appears to me, is a decided step in
advance in the solution of the problem.1

1 As this argument is a rather difficult one to follow, while its theoretical
importance is very great, I add here the following briefer exposition of it, in a
series of propositions ; being, with a few verbal alterations, a copy of what I
wrote on the subject about twenty years back. Some readers may find this
easier to follow than the fuller discussion in the text : —

Can Sterility of Hybrids have been Produced by Natural Selection ?

1. Let there be a species which has varied into two forms each adapted to
certain existing conditions better than the parent form, which they soon
supplant.

2. If these two forms, which are supposed to coexist in the same
district, do not intercross, natural selection will accumulate all favourable
variations till they become well suited to their conditions of life, and form
two slightly differing species.

3. But if these tivo forms freely intercross with each other, and produce
hybrids, which are also quite fertile inter se, then the formation of the two
distinct races or species will be retarded, or perhaps entirely prevented ; for
the offspring of the crossed unions will be more vigorous owing to the cross,
although less adapted to their conditions of life than either of the pure
breeds.

4. Now, let a partial sterility of the hybrids of some considerable propor-
tion of these two forms arise ; and, as this would probably be due to some
special conditions of life, we may fairly suppose it to arise in some definite
portion of the area occupied by the two forms.

5. The result will be that, in that area, the hybrids (although continually
produced by first crosses almost as freely as before) will not themselves
increase so rapidly as the two jmre forms ; and as the two pure forms are, by
the terms of the problem, better suited to their several conditions of life than
the hybrids, they will inevitably increase more rapidly, and will continually
tend to supplant the hybrids altogether at every recurrent severe struggle for
existence.

6. We may fairly suppose, also, that as soon as any sterility appears some
disinclination to cross unions will appear, and this will further tend to the
diminution of the production of hybrids.

180 DARWINISM chap.

Physiological Selection.

Another form of infertility has been suggested by Professor
G. J. Eomanes as having aided in bringing about the char-
acteristic infertility or sterility of hybrids. It is founded on
the fact, already - noticed, that certain individuals of some
species possess what may be termed selective sterility — that is,
while fertile with some individuals of the species they are
sterile with others, and this altogether independently of any
differences of form, colour, or structure. The phenomenon,
in the only form in which it has been observed, is that of "in-
fertility or absolute sterility between two individuals, each of
which is perfectly fertile with all other individuals;" but Mr.
Eomanes thinks that "it would not be nearly so remarkable, or
physiologically improbable, that such incompatibility should run
through a whole race or strain."1 Admitting that this may be

7. In the other part of the area, however, where hybridism occurs with
perfect freedom, hybrids of various degrees may increase till they equal or
even exceed in number the pure species — that is, the incipient species will
be liable to be swamped by intercrossing.

8. The first result, then, of a partial sterility of crosses appearing in one
part of the area occupied by the two forms, will be — that the great majority
of the individuals will there consist of the two pure forms only, while in the
remaining part these will be in a minority, — which is the same as saying that
the new physiological variety of the two forms will be better suited to the
conditions of existence than the remaining portion which has not varied
physiologically.

9. But when the struggle for existence becomes severe, that variety which
is best adapted to the conditions of existence always supplants that which is
imperfectly adapted ; therefore, by natural selection the varieties which are
sterile when crossed Avill become established as the only ones.

10. Now let variations in the amount of sterility and in the disinclination
to crossed unions continue to occur — also in certain parts of the area : exactly
the same result must recur, and the progeny of this new physiological variety
will in time occupy the whole area.

11. There is yet another consideration that would facilitate the process.
It seems probable that the sterility variations would, to some extent, concur
with, and perhaps depend upon, the specific variations ; so that, just in propor-
tion as the two forms diverged and became better adapted to the conditions of
existence, they would become more sterile when intercrossed. If this were
the case, then natural selection would act with double strength ; and those
which were better adapted to survive both structurally and physiologically
would certainly do so.

1 Cases of this kind are referred to at p. 155. It must, however, be noted,
that such sterility in first crosses appears to be equally rare between different
species of the same genus as between individuals of the same species. Mules
and other hybrids are freely produced between very distinct species, but are

vii ON" THE INFERTILITY OF CROSSES 181

so, though we have at present no evidence whatever in
support of it, it remains to be considered whether such physio-
logical varieties could maintain themselves, or whether, as in
the cases of sporadic infertility already discussed, they would
necessarily die out unless correlated with useful characters.
Mr. Romanes thinks that they would persist, and urges that
" Avhenever this one kind of variation occurs it cannot escape
the preserving agency of physiological selection. Hence, even
if it be granted that the variation which affects the re-
productive system in this particular way is a variation of
comparatively rare occurrence, still, as it must always be
preserved whenever it does occur, its influence in the manu-
facture of specific types must be cumulative." The very positive
statements which I have italicised would lead most readers to
believe that the alleged fact had been demonstrated by a
careful working out of the process in some definite supposed
cases. This, however, has nowhere been done in Mr. Romanes'
paper ; and as it is the vital theoretical point on which any
possible value of the new theory rests, and as it appears so
opposed to the self-destructive effects of simple infertility,
which we have already demonstrated when it occurs between
the intermingled portion of two varieties, it must be carefully
examined. In doing so, I will suppose that the required
variation is not of "rare occurrence," but of considerable
amount, and that it appears afresh each year to about the
same extent, thus giving the theory every possible advantage.
Let us then suppose that a given species consists of 100,000
individuals of each sex, with only the usual amount of
fluctuating external variability. Let a physiological variation
arise, so that 10 per cent of the whole number — 10,000
individuals of each sex — while remaining fertile inter se
become quite sterile with the remaining 90,000. This
peculiarity is not correlated Avith any external differences of

themselves infertile or quite sterile ; and it is this infertility or sterility of the
hybrids that is the characteristic — and was once thought to be the criterion
— of species, not the sterility of their first crosses. Hence we should not
expect to find any constant infertility in the first crosses between the distinct
strains or varieties that formed the starting-point of new species, but only a
slight amount of infertility in their mongrel offspring. It follows, that Mr.
Romanes' theory of Physiological Selection — which assumes sterility or in-
fertility between first crosses as the fundamental fact in the origin of species
— does not accord with the general phenomena of hybridism in nature.

182 DARWINISM

form or colour, or with inherent peculiarities of likes or
dislikes leading to any choice as to the pairing of the two sets
of individuals. We have now to inquire, What would he the
result 1

Taking, first, the 10,000 pairs of the physiological or
abnormal variety,- we find that each male of these might
pair Avith any one of the whole 100,000 of the opposite
sex. If, therefore, there was nothing to limit their choice
to particular individuals of either variety, the probabilities
are that 9000 of them would pair with the opposite variety,
and only 1000 with their own variety — that is, that 9000
would form sterile unions, and only one, thousand would form
fertile unions.

Taking, next, the 90,000 normal individuals of either sex,
we find, that each male of these has also a choice of 100,000
to pair with. The probabilities are, therefore, that nine-
tenths of them — that is, 81,000 — would pair with their
normal fellows, while 9000 would pair with the opposite
abnormal variety forming the above-mentioned sterile unions.

Now, as the number of individuals forming a species
remains constant, generally speaking, from year to year, we
shall have next year also 100,000 pairs, of which the two
physiological varieties will be in the proportion of eighty-one
to one, or 98,780 pairs of the normal variety to 1220 1 of
the abnormal, that being the proportion of the fertile unions
of each. In this year we shall find, by the same rule of
probabilities, that only 15 males of the abnormal variety will
pair with their like and be fertile, the remaining 1205 forming
sterile unions with some of the normal variety. The follow-
ing year the total 100,000 pairs will consist of 99,984 of the
normal, and only 1 6 of the abnormal variety ; and the prob-
abilities, of course, are, that the whole of these latter will
pair with some of the enormous preponderance of normal
individuals, and, their unions being sterile, the physiological
variety will become extinct in the third year.

If now in the second and each succeeding year a similar

proportion as at first (10 per cent) of the physiological variety

is produced afresh from the ranks of the normal variety, the

same rate of diminution Avill go on, and it will be found that,

1 The exact number is 1219 -51, but the fractious are omitted for clearness.

vir ON THE INFERTILITY OF CROSSES 183

on the most favourable estimate, the physiological variety can
never exceed 12,000 to the 88,000 of the normal form of the
species, as shown by the following table : — ■

1st Year. 10,000 of physiological variety to 90,000 of normal variety.
2d „ 1,220 + 10,000 again produced.

3d „ 16 + 1,220 + 10,000 do. = 11,236

4th „ 0+ 16+ 1,220 + 10,000 do. =11,236

5th „ 0 + 16 + 1,220 + 10,000 = 11,236

and so on for any number of generations.

In the preceding discussion we have given the theory the
advantage of the large proportion of 10 per cent of this very
exceptional variety arising in its midst year by year, and we
have seen that, even under these favourable conditions, it is
unable to increase its numbers much above its starting-point,
and that it remains wholly dependent on the continued
renewal of the variety for its existence beyond a few years.
It appears, then, that this form of inter -specific sterility
cannot be increased by natural or any other known form of
selection, but that it contains within itself its own principle
of destruction. If it is proposed to get over the difficulty by
postulating a larger percentage of the variety annually arising
within the species, we shall not affect the law of decrease until
we approach equality in the numbers of the two varieties.
But with any such increase of the physiological variety the
species itself would inevitably suffer by the large propor-
tion of sterile unions in its midst, and would thus be at a
great disadvantage in competition with other species which
were fertile throughout. Thus, natural selection will always
tend to weed out any species with too great a tendency to
sterility among its own members, and will therefore prevent
such sterility from becoming the general characteristic of vary-
ing species, which this theory demands should be the case.

On the whole, then, it appears clear that no form of
infertility or sterility between the individuals of a species,
can be increased by natural selection unless correlated with
some useful variation, while all infertility not so correlated
has a constant tendency to effect its own elimination. But
the opposite property, fertility, is of vital importance to every
species, and gives the offspring of the individuals which
possess it, in consequence of their superior numbers, a greater

184 DARWINISM

chance of survival in the hattle of life. It is, therefore,
directly under the control of natural selection, Avhich acts
both by the self-preservation of fertile and the self-destruction
of infertile stocks — except always where correlated as above,
when they become useful, and therefore subject to be increased
by natural selection.

Summary and Concluding Remarks on Hy aridity.

The facts which are of the greatest importance to a com-
prehension of this very difficult subject are those which show
the extreme susceptibility of the reproductive system both in
plants and animals. We have seen how both these classes of
organisms may be rendered infertile, by a change of conditions
which does not affect their general health, by captivity, or
by too close interbreeding. We have seen, also, that infertility
is frequently correlated with a difference of colour, or with other
characters ; that it is not proportionate to divergence of
structure ; that it varies in reciprocal crosses between pairs of
the same species ; while in the cases of dimorphic and tri-
morphic plants the different crosses between the same pair
of individuals may be fertile or sterile at the same time. It
appears as if fertility depended on such a delicate adjustment
of the male and female elements to each other, that, unless
constantly kept up by the jDreservation of the most fertile
individuals, sterility is always liable to arise. This preservation
always occurs Avithin the limits of each species, both because
fertility is of the highest importance to the continuance of the
race, and also because sterility (and to a less extent infertility)
is self-destructive as well as injurious to the species.

So long therefore as a species remains undivided, and in
occupation of a continuous area, its fertility is kept up by
natural selection ; but the moment it becomes separated,
either by geographical or selective isolation, or by diversity
of station or of habits, then, while each portion must be kept
fertile inter se, there is nothing to prevent infertility arising
between the two separated portions. As the tAvo portions
will necessarily exist under someAvhat different conditions of
life, and will usually have acquired some diversity of form and
colour — both which circumstances Ave knoAV to be either the
cause of infertility or to be correlated Avith it, — the fact of

vii ON THE INFERTILITY OF CROSSES 185

some degree of infertility usually appearing between closely
allied but locally or physiologically segregated species is exactly
what we should expect.

The reason why varieties do not usually exhibit a similar
amount of infertility is not difficult to explain. The popular
conclusions on this matter have been drawn chiefly from what
occurs among domestic animals, and we have seen that the
very first essential to their becoming domesticated was that
they should continue fertile under changed conditions of life.
During the slow process of the formation of new varieties by
conscious or unconscious selection, fertility has always been
an essential character, and has thus been invariably preserved
or increased ; while there is some evidence to show that
domestication itself tends to increase fertility.

Among plants, wild species and varieties have been more
frequently experimented on than among animals, and we
accordingly find numerous cases in which distinct species of
plants are perfectly fertile when crossed, their hybrid offspring
being also fertile inter se. We also find some few examples of
the converse fact — varieties of the same species which when
crossed are infertile or even sterile.

The idea that either infertility or geographical isolation is
absolutely essential to the formation of new species, in order
to prevent the swamping effects of intercrossing, has been
shown to be unsound, because the varieties or incipient
species will, in most cases, be sufficiently isolated by
having adopted different habits or by frequenting different
stations ; while selective association, which is known to be
general among distinct varieties or breeds of the same species,
will produce an effective isolation even when the two forms
occupy the same area.

From the various considerations now adverted to, Mr.
Darwin arrived at the conclusion that the sterility or in-
fertility of species with each other, whether manifested in the
difficulty of obtaining first crosses between them or in the
sterility of the hybrids thus obtained, is not a constant or
necessary result of specific difference, but is incidental on
unknown peculiarities of the reproductive system. These
peculiarities constantly tend to arise under changed conditions
owing to the extreme susceptibility of that system, and they

186 DARWINISM chap, vii

are usually correlated with variations of form or of colour.
Hence, as fixed differences of form and colour, slowly gained
by natural selection in adaptation to changed conditions, are
what essentially characterise distinct species, some amount of
infertility between species is the usual result.

Here the problem was left by Mr. Darwin ; but we have
shown that its solution may be carried a step further. If we
accept the association of some degree of infertility, however
slight, as a not unfrequent accompaniment of the external
differences which always arise in a state of nature between
varieties and incipient species, it has been shown that natural
selection has power to increase that infertility just as it has
power to increase other favourable variations. Such an in-
crease of infertility will be beneficial, whenever new species arise
in the same area with the parent form ; and we thus see
how, out of the fluctuating and very unequal amounts of infer-
tility correlated with physical variations, there may have
arisen that larger and more constant amount which appears
usually to characterise well-marked species.

The great body of facts of which a condensed account has
been given in the present chapter, although from an experi-
mental point of view very insufficient, all point to the general
conclusion Ave have now reached, and afford us a not unsatis-
factory solution of the great problem of hybridism in relation
to the origin of species by means of natural selection. Further
experimental research is needed in order to complete the
elucidation of the subject ; but until these additional facts are
forthcoming no new theory seems required for the explanation
of the phenomena.

CHAPTER VIII

THE ORIGIN AND USES OF COLOUR IN ANIMALS

The Darwinian theory threw new light on organic colour — The problem to
be solved — The constancy of animal colour indicates utility — Colour
and environment — Arctic animals white — Exceptions prove the rule —
Desert, forest, nocturnal, and oceanic animals — General theories of
animal colour — Variable protective colouring — Mr. Poulton's experi-
ment's— Special or local colour adaptations — Imitation of particular
objects — How they have been produced — Special protective colouring
of butterflies — Protective resemblance among marine animals — Pro-
tection by terrifying enemies — Alluring coloration — The coloration
of birds' eggs — Colour as a means of recognition — Summary of the
preceding exposition — Influence of locality or of climate on colour —
Concluding remarks.

Among the numerous applications of the Darwinian theory
in the interpretation of the complex phenomena presented by
the organic world, none have been more successful, or are more
interesting, than those which deal with the colours of animals
and plants. To the older school of naturalists colour was a
trivial character, eminently unstable and untrustworthy in the
determination of species; and it appeared to have, in most cases,
no use or meaning to the objects which displayed it. The
bright and often gorgeous coloration of insect, bird, or flower,
was either looked upon as having been created for the enjoy-
ment of mankind, or as due to unknown and perhaps undis-
coverable laws of nature.

But the researches of Mr. Darwin totally changed our point
of view in this matter. He showed, clearly, that some of the
colours of animals are useful, some hurtful to them ; and he
believed that many of the most brilliant colours were developed
by sexual choice ; while his great general principle, that all

188 DARWINISM

the fixed characters of organic beings have been developed
under the action of the law of utility, led to the inevitable
conclusion that so remarkable and conspicuous a character as
colour, which so often constitutes the most obvious distinction
of species from species, or group from group, must also have
arisen from survival of the fittest, and must, therefore, in most
cases have some relation to the wellbeing of its possessors.
Continuous observation and research; carried on by multitudes
of observers during the last thirty years, have shown this to
be the case ; but the problem is found to be far more complex
than was at first supposed. The modes in which colour is of
use to different classes of organisms is very varied, and have
probably not yet been all discovered ; while the infinite variety
and marvellous beauty of some of its developments are such
as to render it hopeless to arrive at a complete and satisfactory
explanation of every individual case. So much, however, has
been achieved, so many curious facts have been explained, and
so much light has been thrown on some of the most obscure
phenomena of nature, that the subject deserves a prominent
place in any account of the Darwinian theory.

The Problem to be Solved.

Before dealing with the various modifications of colour in
the animal world it is necessary to say a few words on colour
in general, on its prevalence in nature, and how it is that the
colours of animals and plants require any special explanation.
What we term colour is a subjective phenomenon, due to the
constitution of our mind and nervous system; while, objectively,
it consists of light- vibrations of different wave-lengths emitted
by, or reflected from, various objects. Every visible object
must be coloured, because to be visible it must send rays of
light to our eye. The kind of light it sends is modified by the
molecular constitution or the surface texture of the object.
Pigments absorb certain rays and reflect the remainder, and
this reflected portion has to our eyes a definite colour, according
to the portion of the rays constituting white light which are
absorbed. Interference colours are produced either by thin
films or by very fine striae on the surfaces of bodies, which
cause rays of certain wave-lengths to neutralise each other,
leaving the remainder to produce the effects of colour. Such

viii ORIGIN AND USES OF COLOUR IN ANIMALS 189

are the colours of soap-bubbles, or of steel or glass on which
extremely fine lines have been ruled ; and these colours often
produce the effect of metallic lustre, and are the cause of most
of the metallic hues of birds and insects.

As colour thus depends on molecular or chemical constitution
or on the minute surface texture of bodies, and, as the matter
of which organic beings are composed consists of chemical com-
pounds of great complexity and extreme instability, and is also
subject to innumerable changes during growth and development,
Ave might naturally expect the phenomena of colour to be more
varied here than in less complex and more stable compounds.
Yet even in the inorganic world we find abundant and varied
colours ; in the earth and in the water ; in metals, gems, and
minerals ; in the sky and in the ocean ; in sunset clouds and in
the many-tinted rainbow. Here we can have no question of
use to the coloured object, and almost as little perhaps in the
vivid red of blood, in the brilliant colours of red snow and
other low algae and fungi, or even in the universal mantle of
green which clothes so large a portion of the earth's surface.
The presence of some colour, or even of many brilliant colours,
in animals and plants would require no other explanation than
does that of the sky or the ocean, of the ruby or the emerald
— that is, it would require a purely physical explanation
only. It is the wonderful individuality of the colours of animals
and plants that attracts our attention — the fact that the colours
are localised in definite patterns, sometimes in accordance with
structural characters, sometimes altogether independent of
them ; while often differing in the most striking and fantastic
manner in allied species. We are thus compelled to look
upon colour not merely as a physical but also as a biological
characteristic, which has been differentiated and specialised
by natural selection, and must, therefore, find its explanation
in the principle of adaptation or utility.

The Constancy of Animal Colour indicates Utility.

That the colours and markings of animals have been
acquired under the fundamental law of utility is indicated by
a general fact which has received very little attention. As a
rule, colour and marking are constant in each species of wild
animal, while, in almost every domesticated animal, there arises

190 DARWINISM

great variability. We see this in our horses and cattle, our
dogs and cats, our pigeons and poultry. Now, the essential
difference between the conditions of life of domesticated and
wild animals is, that the former are protected by man, while
the latter have to protect themselves. The extreme variations
in colour that immediately arise under domestication indicate
a tendency to vary in this way, and the occasional occurrence
of white or piebald or other exceptionally coloured individuals
of many species in a state of nature, shows that this tendency
exists there also ; and, as these exceptionally coloured in-
dividuals rarely or never increase, there must be some con-
stant power at work to keep it in check. This power can
only be natural selection or the survival of the fittest, which
again implies that some colours are useful, some injurious, in
each particular case. With this principle as our guide, let
us see how far we can account both for the general and
special colours of the animal world.

Colour and Environment.

The fact that first strikes us in our examination of the
colours of animals as a whole, is the close relation that exists
between these colours and the general environment. Thus,
white prevails among arctic animals ; yellow or brown in desert
species ; while green is only a common colour in tropical ever-
green forests. If we consider these cases somewhat carefully
we shall find, that they afford us excellent materials for forming
a judgment on the various theories that have been suggested
to account for the colours of the animal world.

In the arctic regions there are a number of animals which are
wholly white all the year round, or which only turn white in
winter. Among the former are the polar bear and the American
polar hare, the snowy owl and the Greenland falcon ; among
the latter the arctic fox, the arctic hare, the ermine, and the
ptarmigan. Those which are permanently white remain among
the snow nearly all the year round, while those which change
their colour inhabit regions which are free from snow in
summer. The obvious explanation of this style of coloration
is, that it is protective, serving to conceal the herbivorous species
from their enemies, and enabling carnivorous animals to approach
their prey unperceived. Two other explanations have, how-

vin ORIGIN AND USES OF COLOUR IN ANIMALS 191

ever, been suggested. One is, that the prevalent white of the
arctic regions has a direct effect in producing the white colour
in animals, either by some photographic or chemical action on
the skin or by a reflex action through vision. The other is,
that the white colour is chiefly beneficial as a means of checking
radiation and so preserving animal heat during the severity of
an arctic winter. The first is part of the general theory that
colour is the effect of coloured light on the objects — a pure
hypothesis which has, I believe, no facts whatever to support
it. The second suggestion is also an hypothesis merely,
since it has not been proved by experiment that a white
.colour, per se, independently of the fur or feathers which is so
coloured, has any effect whatever in checking the radiation of
low-grade heat like that of the animal body. But both alike
are sufficiently disproved by the interesting exceptions to the
rule of white coloration in the arctic regions, which exceptions
are, nevertheless, quite in harmony with the theory of pro-
tection.

Whenever we find arctic animals which, from whatever
cause, do not require protection by the white colour, then
neither the cold nor the snow-glare has any effect upon their
coloration. The sable retains its rich brown fur throughout
the Siberian winter ; but it frequents trees at that season and
not only feeds partially on fruits or seeds, but is able to
catch birds among the branches of the fir-trees, with the bark
of which its colour assimilates. Then we have that thoroughly
arctic animal, the musk-sheep, which is brown and conspicuous ;
but this animal is gregarious, and its safety depends on its
association in small herds. It is, therefore, of more im-
portance for it to be able to recognise its kind at a distance
than to be concealed from its enemies, against which it can
well protect itself so long as it keeps together in a compact
body. But the most striking example is that of the common
raven, which is a true arctic bird, and is found even in
mid -winter as far north as any known bird or mammal.
Yet it always retains its black coat, and the reason, from our
point of view, is obvious. The raven is a powerful bird
and fears no enemy, while, being a carrion-feeder, it has no
need for concealment in order to approach its prey. The
colour of the raven and of the musk -sheep are, therefore,

192 DARWINISM

both inconsistent with any other theory than that the white
colour of arctic animals has been acquired for concealment,
and to that theory both afford a strong support. Here we
have a striking example of the exception proving the rule.

In the desert regions of the earth we find an even more
general accordance of colour with surroundings. The lion,
the camel, and all the desert antelopes have more or less the
colour of the sand or rock among which they live. The
Egyptian cat and the Pampas cat are sandy or earth coloured.
The Australian kangaroos are of similar tints, and the
original colour of the wild horse is supposed to have been
sandy or clay coloured. Birds are equally well protected
by assimilative hues ; the larks, quails, goatsuckers, and
grouse which abound in the North African and Asiatic deserts
are all tinted or mottled so as closely to resemble the average
colour of the soil in the districts they inhabit. Canon
Tristram, who knows these regions and their natural history
so well, says, in an often quoted passage : "In the desert,
where neither trees, brushwood, nor even undulations of
the surface afford the slightest protection to its foes, a
modification of colour which shall be assimilated to that of
the surrounding country is absolutely necessary. Hence,
without exception, the upper plumage of every bird, whether
lark, chat, sylvain, or sand-grouse, and also the fur of all the
smaller mammals, and the skin of all the snakes and lizards,
is of one uniform isabelline or sand colour."

Passing on to the tropical regions, it is among their
evergreen forests alone that we find whole groups of birds
whose ground colour is green. Parrots are very generally
green, and in the East we have an extensive group of green
fruit-eating pigeons ; while the barbets, bee -eaters, turacos,
leaf-thrushes (Phyllornis), white-eyes (Zosterops), and many
other groups, have so much green in their plumage as to tend
greatly to their concealment among the dense foliage. There
can be no doubt that these colours have been acquired as a
protection, when we see that in all the temperate regions,
where the leaves are deciduous, the ground colour of the
great majority of birds, especially on the upper surface, is a
rusty brown of various shades, well corresponding with the
bark, withered leaves, ferns, and bare thickets among which

via ORIGIN AND USES OF COLOUR IN ANIMALS 193

they live in autumn and winter, and especially in early spring
when so many of them build their nests.

Nocturnal animals supply another illustration of the same
rule, in the dusky colours of mice, rats, bats, and moles, and in
the soft mottled plumage of owls and goatsuckers which,
while almost equally inconspicuous in the twilight, are such as
to favour their concealment in the daytime.

An additional illustration of general assimilation of colour
to the surroundings of animals, is furnished by the inhabitants
of the deep oceans. Professor Moseley of the Challenger
Expedition, in his British Association lecture on this subject,
says : " Most characteristic of pelagic animals is the almost
crystalline transparency of their bodies. So perfect is this trans-
parency that very many of them are rendered almost entirely
invisible when floating in the water, while some, even when
caught and held up in a glass globe, are hardly to be seen.
The skin,' nerves, muscles, and other organs are absolutely
hyaline and transparent, but the liver and digestive tract
often remain opaque and of a yellow or brown colour, and
exactly resemble when seen in the water small pieces of
floating seaweed." Such marine organisms, however, as
are of larger size, and either occasionally or habitually float
on the surface, are beautifully tinged with blue above, thus
harmonising with the colour of the sea as seen by hovering
birds ; while they are white below, and are thus invisible
against the wave-foam and clouds as seen by enemies beneath
the surface. Such are the tints of the beautiful nudibranchiate
mollusc, Glaucus atlanticus, and many others.

General Theories of Animal Colour.

We are now in a position to test the general theories, or,
to speak more correctly, the popular notions, as to the origin
of animal coloration, before proceeding to apply the principle
of utility to the explanation of some among the many
extraordinary manifestations of colour in the animal world.
The most generally received theory undoubtedly is, that
brilliancy and variety of colour are due to the direct action
of light and heat ; a theory no doubt derived from the
abundance of bright - coloured birds, insects, and flowers
which are brought from tropical regions. There are, however,

0

194 DARWINISM

two strong arguments against this theory. We have already-
seen how generally bright coloration is wanting in desert
animals, yet here heat and light are both at a maximum,
and if these alone were the agents in the production of
colour, desert animals should be the most brilliant. Again,
all naturalists who have lived in tropical regions know that
the proportion of bright to dull coloured species is little if
any greater there than in the temperate zone, while there are
many tropical groups in which bright colours are almost en-
tirely unknown. No part of the world presents so many
brilliant birds as South America, yet there are extensive
families, containing many hundreds of species, which are as
plainly coloured as our average temperate birds. Such are the
families of the bush-shrikes and ant-thrushes (Formicariidae),
the tyrant-shrikes (Tyrannidse), the American creepers (Den-
dfocolaptidse), together with a large proportion of the wood-
warblers (Mniotiltidse), the finches, the wrens, and some other
groups. In the eastern hemisphere, also, Ave have the babbling-
thrushes (Timaliidse), the cuckoo-shrikes (Campephagida?), the
honey-suckers (Meliphagidae), and several other smaller groups
which are certainly not coloured above the average standard
of temperate birds.

Again, there are many families of birds which spread over
the whole world, temperate and tropical, and among these the
tropical species rarely present any exceptional brilliancy of
colour. Such are the thrushes, goatsuckers, hawks, plovers,
and ducks ; and in the last-named group it is the temperate
and arctic zones that afford the most brilliant coloration.

The same general facts are found to prevail among insects.
Although tropical insects present some of the most gorgeous
coloration in the whole realm of nature, yet there are
thousands and tens of thousands of species which are as dull
coloured as any in our cloudy land. The extensive family of
the carnivorous ground-beetles (Carabidse) attains its greatest
brilliancy in the temperate zone ; while by far the larger
proportion of the great families of the longicorns and the
weevils, are of obscure colours even in the tropics. In butter-
flies, there is undoubtedly a larger proportion of brilliant
colour in the tropics ; but if we compare families which are
almost equally developed over the globe — as the Pieridoe or

viii ORIGIN AND USES OF COLOUR IN ANIMALS 195

whites and yellows, and the Satyridse or ringlets — we shall find
no great disproportion in colour between those of temperate
and tropical regions.

The various facts which have now briefly been noticed are
sufficient to indicate that the light and heat of the sun are
not the direct causes of the colours of animals, although they
may favour the production of colour when, as in tropical
regions, the persistent high temperature favours the develop-
ment of the maximum of life. We will now consider the
next suggestion, that light reflected from surrounding coloured
objects tends to produce corresponding colours in the animal
world.

This theory is founded on a number of very curious facts
which prove, that such a change does sometimes occur and is
directly dependent on the colours of surrounding objects ; but
these facts are comparatively rare and exceptional in their
nature, and the same theory will certainly not apply to the in-
finitely varied colours of the higher animals, many of which
are exposed to a constantly varying amount of light and
colour during their active existence. A brief sketch of these
dependent changes of colour may, however, be advantageously
given here.

Variable Protective Colouring.

There are two distinct kinds of change of colour in animals
due to the colouring of the environment. In one case the
change is caused by reflex action set up by the animal seeing
the colour to be imitated, and the change produced can be
altered or repeated as the animal changes its position. In the
other case the change occurs but once, and is probably not
due to any conscious or sense action, but to some direct in-
fluence on the surface tissues while the creature is undergoing
a moult or change to the pupa form.

The most striking example of the first class is that of the
chameleon, which changes to white, brown, yellowish, or
green, according to the colour of the object on which it rests.
This change is brought about by means of two layers of
pigment cells, deeply seated in the skin, and of bluish and
yellowish colours. By suitable muscles these cells can be
forced upwards so as to modify the colour of the skin, which,

196 DARWINISM chap.

when they are not brought into action, is a dirty white.
These animals are excessively sluggish and defenceless, and the
power of changing their colour to that of their immediate sur-
roundings is no doubt of great service to them. Many of the
flatfish are also capable of changing their colour according to
the colour of the bottom they rest on ; and frogs have a
similar power to a limited extent. Some Crustacea also
change colour, and the power is much developed in the
Chameleon shrimp (Mysis Chamaeleon) which is gray when on
sand, but brown or green when among brown or green seaweed.
It has been proved by experiment that when this animal is
blinded the change does not occur. In all these cases,
therefore, we have some form of reflex or sense action by
which the change is produced, probably by means of pigment
cells beneath the skin as in the chameleon.

The second class consists of certain larvse, and pupae, which
undergo changes of colour when exposed to differently
coloured surroundings. This subject has been carefully
investigated by Mr. E. B. Poulton, who has communicated
the results of his experiments to the Royal Society.1 It had
been noticed that some species of larvse which fed on several
different plants had colours more or less corresponding to the
particular plant the individual fed on. Numerous cases are
given in Professor Meldola's article on "Variable Protective
Colouring" (Proc. Zool. Soc, 1873, p. 153), and while the
general green coloration was attributed to the presence of
chlorophyll beneath the skin, the particular change in corre-
spondence to each food-plant was attributed to a special
function which had been developed by natural selection.
Later on, in a note to his translation of Weissmann's Theory
of Descent, Professor Meldola seemed disposed to think that
the variations of colour of some of the species might be
phytophagic — that is, due to the direct action of the differently
coloured leaves on which the insect fed. Mr. Poulton's
experiments have thrown much light on this question, since he
has conclusively proved that, in the case of the sphinx cater-
pillar of Smerinthus ocellatus, the change of colour is not due
to the food but to the coloured light reflected from the leaves.

1 Proceedings of the Royal Society, No. 243, 1SS6 ; Transactions of the Royal
Society, vol. clxxviii. B. pp. 311-441.

vnr ORIGIN AND USES OF COLOUR IN ANIMALS 197

This was shown by feeding two sets of larvae on the same
plant but exposed to differently coloured surroundings,
obtained by sewing the leaves together, so that in one case
only the dark upper surface, in the other the whitish under
surface was exposed to view. The result in each case was a
corresponding change of colour in the larvae, confirming the
experiments on different individuals of the same batch of
larvae which had been supplied with different food-plants or
exposed to a different coloured light.

An even more interesting series of experiments was made
on the colours of pupae, which in many cases were known to
be affected by the material on which they underwent their
transformations. The late Mr. T. W. Wood proved, in 1867,
that the pupae of the common cabbage butterflies (Pieris
brassicae and P. rapae) were either light, or dark, or green, ac-
cording to the coloured boxes they were kept in, or the colours
of the fences, walls, etc., against which they were suspended.
Mrs. Barber in South Africa found that the pupae of Papilio
Nireus underwent a similar change, being deep green when
attached to orange leaves of the same tint, pale yellowish-green
when on a branch of the bottle-brush tree whose half-dried
leaves were of this colour, and yellowish when attached to
the wooden frame of a box. A few other observers noted
similar phenomena, but nothing more was done till Mr.
Poulton's elaborate series of experiments with the larvae of
several of our common butterflies were the means of clearing
up several important points. He showed that the action
of the coloured light did not affect the pupa itself but the
larva, and that only for a limited period of time. After
a caterpillar has done feeding it wanders about seeking a
suitable place to undergo its transformation. When this is
found it rests quietly for a day or two, spinning the web from
which it is to suspend itself ; and it is during this period of
quiescence, and perhaps also the first hour or two after its
suspension, that the action of the surrounding coloured
surfaces determines, to a considerable extent, the colour of
the pupa. By the application of various surrounding colours
during this period, Mr. Poulton was able to modify the colour
of the pupa of the common tortoise-shell butterfly from nearly
black to pale, or to a brilliant golden ; and that of Pieris rapae

198 DARWINISM

from dusky through pinkish to pale green. It is interesting
to note, that the colours produced were in all cases such only
as assimilated with the surroundings usually occupied by the
species, and also, that colours which did not occur in such sur-
roundings, as dark red or blue, only produced the same effects
as dusky or black.

Careful experiments were made to ascertain whether the
effect was produced through the sight of the caterpillar. The
ocelli were covered with black varnish, but neither this, nor
cutting off the spines of the tortoise-shell larva to ascertain
whether they might be sense-organs, produced any effect on
the resulting colour. Mr. Poulton concludes, therefore, that
the colour-action probably occurs over the whole surface of
the body, setting up physiological processes which result in
the corresponding colour-change of the pupa. Such changes
are, however, by no means universal, or even common, in
protectively coloured pupae, since in Papilio machaon and
some others which have been experimented on, both in this
country and abroad, no change can be produced on the pupa
by any amount of exposure to differently coloured surround-
ings. It is a curious point that, with the small tortoise-shell
larva, exposure to light from gilded surfaces produced pupae
with a brilliant golden lustre ; and the explanation is supposed
to be that mica abounded in the original habitat of the species,
and that the pupse thus obtained protection when suspended
against micaceous rock. Looking, however, at the wide range
of the species and the comparatively limited area in which
micaceous rocks occur, this seems a rather improbable ex-
planation, and the occurrence of this metallic appearance is
still a difficulty. It does not, however, commonly occur in
this country in a natural state.

The two classes of variable colouring here discussed are
evidently exceptional, and can have little if any relation to
the colours of those more active creatures which are continu-
ally changing their position with regard to surrounding objects,
and whose colours and markings are nearly constant through-
out the life of the individual, and (with the exception of
sexual differences) in all the individuals of the species. We
will now briefly pass in review the various characteristics and
uses of the colours which more generally prevail in nature ;

vni ORIGIN AND USES OF COLOUR IN ANIMALS 199

and having already discussed those protective colours which
serve to harmonise animals with their general environment,
we have to consider only those cases in which the colour
resemblance is more local or sjDecial in its character.

Special or Local Colour Adaptations.

This form of colour adaptation is generally manifested by
markings rather than by colour alone, and is extremely pre-
valent both among insects and vertebrates, so that we shall
be able to notice only a few illustrative cases. Among our
native birds we have the snipe and woodcock, whose markings
and tints strikingly accord with the dead marsh vegetation
among which they live ; the ptarmigan in its summer dress is
mottled and tinted exactly like the lichens which cover the
stones of the higher mountains ; while young unfledged plovers
are spotted so as exactly to resemble the beach pebbles among
which they crouch for protection, as beautifully exhibited in
one of the cases of British birds in the Natural History
Museum at South Kensington.

In mammalia, we notice the frequency of rounded spots on
forest or tree haunting animals of large size, as the forest
deer and the forest cats ; while those . that frequent reedy or
grassy places are striped . vertically, as the marsh antelopes
and the tiger. I had long been of opinion that the brilliant
yellow and black strijDes of the tiger were adaptive, but have
only recently obtained proof that it is so. An experienced
tiger-hunter, Major Watford, states in a letter, that the haunts
of the tiger are invariably full of the long grass, dry and pale
yellow for at least nine months of the year, which covers the
ground wherever there is water in the rainy season, and he
adds : " I once, while following up a wounded tiger, failed for
at least a minute to see him under a tree in grass at a distance
of about twenty yards — jungle open- — but the natives saw
him, and I eventually made him out well enough to shoot
him, but even then I could not see at what part of him I was
.aiming. There can be no doubt whatever that the colour of
both the tiger and the panther renders them almost invisible,
especially in a strong blaze of light, when among grass, and
one does not seem to notice stripes or spots till they are
dead." It is the black shadows of the vegetation that

200 DARWINISM

assimilate with the Mack stripes of the tiger ; and, in like
manner, the spotty shadows of leaves in the forest so
harmonise with the spots of ocelots, jaguars, tiger-cats, and
spotted deer as to afford them a very perfect concealment.

In some cases the concealment is effected by colour's and
markings which are so striking and peculiar that no one who
had not seen the creature in its native haunts would imagine
them to be protective. An example of this is afforded by the
banded fruit pigeon of Timor, whose pure white head and
neck, black wings and back, yellow belly, and deeply-curved
black band across the breast, render it a very handsome and
conspicuous bird. Yet this is what Mr. H. O. Forbes says of
it : " On the trees the white-headed fruit pigeon (Ptilopus
cinctus) sate motionless during the heat of the clay in numbers,
on well-exposed branches ; but it was with the utmost difficulty
that I or my sharp-eyed native servant could ever detect them,
even in trees where we knew they were sitting." 1 The trees
referred to are species of Eucalyptus which abound in Timor.
They have whitish or yellowish bark and very open foliage,
and it is the intense sunlight casting black curved shadows of
one branch upon another, with the white and yellow bark and
deep blue sky seen through openings of the foliage, that pro-
duces the peculiar combination of colours and shadows to
which the colours and markings of this bird have become so
closely assimilated.

Even such brilliant and gorgeously coloured birds as the
sun-birds of Africa are, according to an excellent observer,
often protectively coloured. Mrs. M. E. Barber remarks
that " A casual observer would scarcely imagine that the
highly varnished and magnificently coloured plumage of the
various sjiecies of Noctarinea could be of service to them, yet
this is undoubtedly the case. The most unguarded moments
of the lives of these birds are those that are spent amongst
the flowers, and it is then that they are less wary than at any
other time. The different species of aloes, which blossom in
succession, form the principal sources of their winter supplies
of food ; and a legion of other gay flowering plants in spring
and summer, the aloe blossoms especially, are all brilliantly
coloured, and they harmonise admirably Avith the gay plumage
1 A Naturalist's Wanderings in the Eastern Archipelago, p. 460.

via ORIGIN AND USES OF COLOUR IN ANIMALS 201

of the different species of sun-birds. Even the keen eye of a
hawk will fail to detect them, so closely do they resemble the
flowers they frequent. The sun-birds are fully aware of this
fact, for no sooner have they relinquished the flowers than they
become exceedingly wary and rapid in flight, darting arrow-
like through the air and seldom remaining in exposed situations.
The black sun-bird (Nectarinea amethystina) is never absent
from that magnificent forest-tree, the ' Kaffir Boom ' (Erythrina
caffra) ; all clay long the cheerful notes of these birds may be
heard amongst its spreading branches, yet the general aspect
of the tree, which consists of a huge mass of scarlet and purple-
black blossoms without a single green leaf, blends and har-
monises with the colours of the black sun-bird to such an extent
that a dozen of them may be feeding amongst its blossoms
without being conspicuous, or even visible." x

Some other cases will still further illustrate how the colours
of even "very conspicuous animals may be adapted to their
peculiar haunts.

The late Mr. Swinhoe says of the Kerivoula picta, which
he observed in Formosa : " The body of this bat was of an
orange colour, but the wings were painted with orange-yellow
and black. It was caught suspended,. head downwards, on a
cluster of the fruit of the longan tree (Nephelium longanum).
Now this tree is an evergreen, and all the year round some
portion of its foliage is undergoing decay, the particular leaves
being, in such a stage, partially orange and black. This bat
can, therefore, at all seasons suspend from its branches and
elude its enemies by its resemblance to the leaves of the
tree."2

Even more curious is the case of the sloths — defenceless
animals which feed upon leaves, and hang from the branches
of trees with their back downwards. Most of the species have
a curious buff-coloured spot on the back, rounded or oval in
shape and often with a darker border, which seems placed
there on purpose to make them conspicuous ; and this was a
great puzzle to naturalists, because the long coarse gray or
greenish hair was evidently like tree-moss and therefore
protective. But an old writer, Baron von Slack, in his Voyage

1 Trans. Phil. Boc. (? of S. Africa), 1878, part iv, p. 27.
2 Proc. Zool. Soc, 1862 p. 357.

202 DARWINISM

to Surinam (1810), had already explained the matter. He
says : " The colour and even the shape of the hair are much
like withered moss, and serve to hide the animal in the trees,
but particularly when it has that orange-coloured spot between
the shoulders and lies close to the tree ; it looks then exactly
like a piece of branch where the rest has been broken off, by
which the hunters are often deceived." Even such a huge
animal as the giraffe is said to be perfectly concealed by its
colour and form when standing among the dead and broken
trees that so often occur on the outskirts of the thickets where
it feeds. The large blotch-like spots on the skin and the
strange shape of the head and horns, like broken branches, so
tend to its concealment that even the keen-eyed natives have
been known to mistake trees for giraffes or giraffes for trees.

Innumerable examples of this kind of protective colouring
occur among insects • beetles mottled like the bark of trees or
resembling the sand or rock or moss on which they live, with
green caterpillars of the exact general tints of the foliage they
feed on ; but there are also many cases of detailed imitation of
particular objects by insects that must be briefly described.1

Protective Imitation of Particular Objects.

The insects which present this kind of imitation most per-
fectly are the Phasmidse, or stick and leaf insects. The well-

1 With reference to this general resemblance of insects to their environment
the following remarks by Mr. Poulton are very instructive. He says :
" Holding the larva of Sphinx ligustri in one hand and a twig of its food-
plant in the other, the wonder we feel is, not at the resemblance but at the
difference ; we are surprised at the difficulty experienced in detecting so con-
spicuous an object. And yet the protection is very real, for the larvae will be
passed over by those who are not accustomed to their appearance, although the
searcher may be told of the presence of a large caterpillar. An experienced
entomologist may also fail to find the larvae till after a considerable search.
This is general protective resemblance, and it depends upon a general harmony
between the appearance of the organism and its whole environment. It is
impossible to understand the force of this protection for any larva, without
seeing it on its food-plant and in an entirely normal condition. The artistic
effect of green foliage is more complex than we often imagine ; numberless
modifications are wrought by varied lights and shadows upon colours which are
in themselves far from uniform. In the larva of Papilio machaon the pro-
tection is very real when the larva is on the food-plant, and can hardly
be appreciated at all when the two are apart." Numerous other examples are
given in the chapter on ' ' Mimicry and other Protective Resemblances among
Animals," in my Contributions to the Theory of Natural Selection.

vrn ORIGIN AND USES OF COLOUR IN ANIMALS 203

known leaf-insects of Ceylon and of Java, species of Phyllium,
are so wonderfully coloured and veined, with leafy expansions
on the legs and thorax, that not one person in ten can see
them when resting on the food-plant close beneath their eyes.
Others resemble pieces of stick with all the minutiae of knots
and branches, formed by the insects' legs, which .are stuck out
rigidly and unsymmetrically. I have often been unable to
distinguish between one of these insects and a real piece of
stick, till I satisfied myself by touching it and found it to be
alive. One species, which was brought me in Borneo, was
covered with delicate semi transparent green foliations, exactly
resembling the hepaticae which cover pieces of rotten stick in
the damp forests. Others resemble dead leaves in all their
varieties of colour and form ; and to show how perfect is the
protection obtained and how important it is to the possessors
of it, the following incident, observed by Mr. Belt in Nicaragua,
is most -instructive. Describing the armies of foraging ants in
the forest which devour every insect they can catch, he says :
" I was much surprised with the behaviour of a green leaf-
like locust. This insect stood immovably among a host of ants,
many of which ran over its legs without ever discovering there
was food within their reach. So fixed was its instinctive
knowledge that its safety depended on its immovability, that
it allowed me to pick it up and replace it among the ants without
making a single effort to escape. This species closely resembles
a green leaf." l

Caterpillars also exhibit a considerable amount of detailed
resemblance to the plants on which they live. Grass -feeders
are striped longitudinally, while those on ordinary leaves are
always striped obliquely. Some very beautiful protective
resemblances are shown among the caterpillars figured in
Smith and Abbott's Lepiclopterous Insects of Georgia, a work
published in the early part of the century, before any theories
of protection were started. The plates in this work are
most beautifully executed from drawings made by Mr. Abbott,
representing the insects, in every case, on the plants which
they frequented, and no reference is made in the descriptions
to the remarkable protective details which appear upon the
plates. We have, first, the larva of Sphinx fuciformis feeding

1 The Naturalist in Nicaragua, p. 1 9.

204 DARWINISM

on a plant with linear grass-like leaves and small blue flowers ;
and we find the insect of the same green as the leaves, striped
longitudinally in accordance with the linear leaves, and with
the head blue corresponding both in size and colour with the
flowers. Another species (Sphinx tersa) is represented feeding
on a plant with small red flowers situated in the axils of the
leaves ; and the larva has a row of seven red spots, unequal
in size, and corresponding very closely with the colour and
size of the flowers. Two other figures of sphinx larvae are
very curious. That of Sphinx pampinatrix feeds on a wild
vine (Vitis indivisa), having green tendrils, and in this species
the curved horn on the tail is green, and closely imitates in
its curve the tip of the tendril. But in another species
(Sphinx cranta), which feeds on the fox -grape (Vitis vulpina),
the horn is very long and red, corresponding with the long red-
tipped tendrils of the plant. Both these larvae are green with
oblique stripes, to harmonise Avith the veined leaves of the
vines ; but a figure is also given of the last-named species after
it has done feeding, when it is of a decided brown colour and
has entirely lost its horn. This is because it then descends to
the ground to bury itself, and the green colour and red
horn would be conspicuous and dangerous ; it therefore loses
both at the last moult. Such a change of colour occurs in
many species of caterpillars. Sometimes the change is seasonal ;
and, in those which hibernate with us, the colour of some
species, which is brownish in autumn in adaptation to the
fading foliage, becomes green in spring to harmonise with the
newly -opened leaves at that season.1

Some of the most curious examples of minute imitation
are afforded by the caterpillars of the geometer moths, which
are always brown or reddish, and resemble in form little
twigs of the plant on which they feed. They have the habit,
when at rest, of standing out obliquely from the branch, to
which they hold on by their hind pair of prolegs or claspers,
and remain motionless for hours. Speaking of these pro-
tective resemblances Mr. Jenner Weir says : " After being
thirty years an entomologist I was deceived myself, and took
out my pruning scissors to cut from a plum tree a spur which
I thought I had overlooked. This turned out to be the larva
1 R. Meldola, in Proc. Zool. Soc, 1873, p. 155.

viii ORIGIN AND USES OF COLOUR IN ANIMALS 205

of a geometer two inches long. I showed it to several members
of my family, and denned a space of four inches in which it
was to be seen, but none of them could perceive that it was a
caterpillar."1

One more example of a protected caterpillar must be
given. Mr. A. Everett, writing from Sarawak, Borneo, says :
" I had a caterpillar brought me, which, being mixed by my
boy with some other things, I took to be a bit of moss with
two exquisite pinky-white seed-capsules ; but I soon saw that
it moved, and examining it more closely found out its real
character : it is covered with hair, with two little pink spots
on the upper surface, the general hue being more green. Its
motions are very slow, and when eating the head is with-
drawn beneath a fleshy mobile hood, so that the action of
feeding does not produce any movement externally. It was
found in the limestone hills at Busan, the situation of all
others where mosses are most plentiful and delicate, and
where they partially clothe most of the protruding masses
of rock."

How these Imitations have been Produced.

To many persons it will seem impossible that such beauti-
ful and detailed resemblances as those now described— - and
these are only samples of thousands that occur in all parts of
the world — can have been brought about by the preservation
of accidental useful variations. But this will not seem so
surprising if we keep in mind the facts set forth in our
earlier chapters — the rapid multiplication, the severe struggle
for existence, and the constant variability of these and
all other organisms. And, further, we must remember
that these delicate adjustments are the result of a process
which has been going on for millions of years, and that we
now see the small percentage of successes among the myriads
of failures. From the very first appearance of insects and
their various kinds of enemies the need of protection arose,
and was usually most easily met by modifications of colour.
Hence, we may be sure that the earliest leaf- eating insects
acquired a green colour as one of the necessities of their
existence ; and, as the species became modified and specialised,

1 Nature, vol. iii. p. 166.

206 DARWINISM

those feeding on particular species of plants would rapidly
acquire the peculiar tints and markings best adapted to
conceal them upon those plants. Then, every little variation
that, once in a hundred years perhaps, led to the preservation
of some larva which was thereby rather better concealed than
its fellows, would form the starting-point of a further
development, leading ultimately to that perfection of imitation
in details which now astonishes us. The researches of Dr.
Weismann illustrate this progressive adaptation. The very
young larvae of several species are green or yellowish without
any markings ; they then, in subsequent moults, obtain certain
markings, some of which are often lost again before the larva
is fully grown. The early stages of those species which,
like elephant hawk -moths (Chserocampa), have the anterior
segments elongated and retractile, Avith large eye -like spots
to imitate the head of a vertebrate, are at first like those of
non-retractile species, the anterior segments being as large as
the rest. After the first moult they become smaller, com-
paratively ; but it is only after the second moult that the
ocelli begin to appear, and these are not fully denned till after
the third moult. This progressive development of the in-
dividual— the ontogeny — -gives us a clue to the ancestral
development of the whole race — the phylogeny ; and we are
enabled to picture to ourselves the very slow and gradual
steps by which the existing perfect adaptation has been
brought about. In many larvae great variability still exists,
and in some there are two or more distinctly-coloured forms
— usually a dark and a light or a brown and a green form.
The larva of the humming-bird hawk -moth (Macroglossa
stellatarum) varies in this manner, and Dr. Weismann raised
five varieties from a batch of eggs from one moth. It feeds
on species of bedstraw (Galium verum and G. mollugo), and
as the green forms are less abundant than the brown, it has
probably undergone some recent change of food -plant or
of habits which renders brown the more protective colour.

Special Protective Colouring of Butterflies.

We will now consider a few cases of special protective
colouring in the perfect butterfly or moth. Mr. Mansel
Weale states that in South Africa there is a great prevalence

vin ORIGIN AND USES OF COLOUR IN ANIMALS 207

of white and silvery foliage or bark, sometimes of dazzling
brilliancy, and that many insects and their larvae have brilliant
silvery tints which are protective, among them being three
species of butterflies whose undersides are silvery, and which
are thus effectually protected when at rest.1 A common
African butterfly (Aterica meleagris) always settles on the
ground with closed wings, which so closely resemble the soil
of the district that it can with difficulty be seen, and the
colour varies with the soil in different localities. Thus
specimens from Senegambia were dull brown, the soil being
reddish sand and iron -clay; those from Calabar and Came-
roons were light brown with numerous small white spots, the
soil of those countries being light brown clay with small
quartz pebbles ; while in other localities where the colours of
the soil were more varied the colours of the butterfly varied
also. Here we have variation in a single species which has
become -specialised in certain areas to harmonise with the
colour of the soil.2

Many butterflies, in all parts of the world, resemble dead
leaves on their under side, but those in which this form
of protection is carried to the greatest perfection are the
species of the Eastern genus Kallima. In India K. inachis,
and in the larger Malay islands K. paralekta, are very com-
mon. They are rather large and showy butterflies, orange
and bluish on the upper side, with a very rapid flight, and
frequenting dry forests. Their habit is to settle always where
there is some dead or decaying foliage, and the shape and
colour of the wings (on the under surface), together with the
attitude of the insect, is such as to produce an absolutely
perfect imitation of a dead leaf. This is effected by the
butterfly always settling on a twig, with the short tail of the
hind wings just touching it and forming the leaf -stalk.
From this a dark curved line runs across to the elongated tip
of the upper wings, imitating the midrib, on both sides of
which are oblique lines, formed partly by the nervures and
partly by markings, which give the effect of the usual veining
of a leaf. The head and antennae fit exactly between the
closed upper wings so as not to interfere with the outline,

1 Trans. Ent. Soc. Lond., 1878, p. 185.
2 Ibid. {Proceedings, p. xlii.)

208 DARWINISM

which has just that amount of irregular curvature that is seen
in dry and withered leaves. The colour is very remarkable
for its extreme amount of variability, from deep reddish-brown
to olive or pale yellow, hardly two specimens being exactly
alike, but all coming within the range of colour of leaves in
various stages of decay. Still more curious is the fact that
the paler wings, which imitate leaves most decayed, are
usually covered with small black dots, often gathered into
circular groups, and so exactly resembling the minute fungi
on decaying leaves that it is hard at first to believe that the
insects themselves are not attacked by some such fungus.
The concealment produced by this wonderful imitation is
most complete, and in Sumatra I have often seen one enter a
bush and then disappear like magic. Once I was so fortunate
as to see the exact spot on which the insect settled ; but even
then I lost sight of it for some time, and only after a per-
sistent search discovered that it was close before my eyes.1
Here we have a kind of imitation, which is very common in a
less developed form, carried to extreme perfection, with the
result that the species is very abundant over a considerable
area of country.

Protective Resemblance among Marine Animals.

Among marine animals this form of protection is very
common. Professor Moseley tells us that all the inhabitants
of the Gulf-weed are most remarkably coloured, for purposes
of protection and concealment, exactly like the weed itself.
" The shrimps and crabs which swarm in the weed are of
exactly the same shade of yellow as the weed, and have white
markings upon their bodies to represent the patches of Mem-
branipora. The small fish, Antennarius, is in the same way
weed-colour with white spots. Even a Planarian worm, which
lives in the weed, is similarly yellow-coloured, and also a
mollusc, Scyllsea pelagica." The same writer tells us that " a
number of little crabs found clinging to the floats of the blue-
shelled mollusc, Ianthina, were all coloured of a corresponding
blue for concealment."2

1 Wallace's Malay Archipelago, vol. i. p. 204 (fifth edition, p. 130), with
figure.

2 Moseley's Notes by a Naturalist on the Challenger.

Vlii ORIGIN AND USES OF COLOUR IN ANIMALS 209

Professor E. S. Morse of Salem, Mass., found that most
of the New England marine mollusca were protectively
coloured ; instancing among others a little red chiton on rocks
clothed with red calcareous algse, and Crepidula plana, liv-
ing within the apertures of the shells of larger species of
Gasteropods and of a pure white colour corresponding to its
habitat, while allied species living on seaweed or on the
outside of dark shells were dark brown.1 A still more
interesting case has been recorded by Mr. George Brady. He
says : " Amongst the JSTullipore which matted together the
laminaria roots in the Firth of Clyde were living numerous
small starfishes (Ophiocoma bellis) which, except when their
writhing movements betrayed them, were quite undistinguish-
able from the calcareous branches of the alga ; their rigid
angularly twisted rays had all the appearance of the coralline,
and exactly assimilated to its dark purple colour, so that
though I- held in my hand a root in which were half a dozen
of the starfishes, I was really unable to detect them until
revealed by their movements."2

These few examples are sufficient to show that the principle
of protective coloration extends to the ocean as well as over
the earth ; and if we consider how completely ignorant we
are of the habits and surroundings of most marine animals, it
may well happen that many of the colours of tropical fishes,
which seem to us so strange and so conspicuous, are really
protective, owing to the number of equally strange and
brilliant forms of corals, sea-anemones, sponges, and sea-
weeds among which they live.

Protection by Terrifying Enemies.

A considerable number of quite defenceless insects obtain
protection from some of their enemies by having acquired a
resemblance to dangerous animals, or by some threatening or
unusual appearance. This is obtained either by a modifica-
tion of shape, of habits, of colour, or of all combined. The
simplest form of this protection is the aggressive attitude of
the caterpillars of the Sphingidae, the forepart of the body

1 Proceedings of the Boston Soc. of Nat. Hist., vol. xiv. 1871.

2 Nature, 1870, p. 376.

P

210 . DARWINISM

being erected so as to produce a rude resemblance to the figure
of a sphinx, hence the name of the family. The protection is
carried further by those species which retract the first three
segments and have large ocelli on each side of the fourth
segment, thus giving to the caterpillar, when the forepart of
its body is elevated, the appearance of a snake in a threaten-
ing attitude.

The blood-red forked tentacle, thrown out of the neck of
the larvae of the genus Papilio when alarmed, is, no doubt, a
protection against the attacks of ichneumons, and may, per-
haps, also frighten small birds ; and the habit of turning up
the tail possessed by the harmless rove-beetles (Staphylinidse),
giving the idea that they can sting, has, probably, a similar
use. Even an unusual angular form, like a crooked twig or
inorganic substance, may be protective ; as Mr. Poulton thinks
is the case with the curious caterpillar of Notodonta ziczac,
which, by means of a few slight protuberances on its body,
is able to assume an angular and very unorganic-looking
appearance. But perhaps the most perfect example of this
kind of protection is exhibited by the large caterpillar of
the Royal Persimmon moth (Bombyx regia), a native of
the southern states of North America, and known there as
the "Hickory -horned devil." It is a large green cater-
pillar, often six inches long, ornamented with an immense
crown of orange-red tubercles, which, if disturbed, it erects
and shakes from side to side in a very alarming manner.
In its native country the negroes believe it to be as deadly
as a rattlesnake, whereas it is perfectly innocuous. The
green colour of the body suggests that its ancestors were
once protectively coloured ; but, growing too large to be
effectually concealed, it acquired the habit of shaking its head
about in order to frighten away its enemies, and ultimately
developed the crown of tentacles as an addition to its terrify-
ing powers. This species is beautifully figured in Abbott and
Smith's Lepidopterous Insects of Georgia.

Alluring Coloration.

Besides those numerous insects which obtain protection
through their resemblance to the natural objects among which
they live, there are some whose disguise is not used for

viii ORIGIN" AND USES OF COLOUR IN ANIMALS 211

concealment, but as a direct means of securing their prey by
attracting them within the enemy's reach. Only a few cases
of this kind of coloration have yet been observed, chiefly
among spiders and mantidse ; but, no doubt, if attention
were given to the subject in tropical countries, many more
would be discovered. Mr. H. 0. Forbes has described a
most interesting example of this kind of simulation in
Java. While pursuing a large butterfly through the jungle,
he was stopped by a dense bush, on a leaf of which he
observed one of the skipper butterflies sitting on a bird's
dropping. "I had often," he says, "observed small Blues
at rest on similar spots on the ground, and have wondered
what such a refined and beautiful family as the Lycaenidse
could find to enjoy, in food apparently so incongruous
for a butterfly. I approached with gentle steps, but
ready net, to see if possible how the present species was
engaged.- It permitted me to get quite close, and even to
seize it between my fingers ; to my surprise, however, part of
the body remained behind, adhering as I thought to the
excreta. I looked closely, and finally touched with my finger
the excreta to find if it were glutinous. To my delighted
astonishment I found that my eyes had been most perfectly
deceived, and that what seemed to be the excreta was a
most artfully coloured spider, lying on its back with its feet
crossed over and closely aclpressed to the body." Mr. Forbes
then goes on to describe the exact appearance of such excreta,
and how the various parts of the spider are coloured to
produce the imitation, even to the liquid jDortion which
usually runs a little down the leaf. This is exactly imitated
by a portion of the thin web which the spider first spins
to secure himself firmly to the leaf ; thus producing, as Mr.
Forbes remarks, a living bait for butterflies and other insects
so artfully contrived as to deceive a pair of human eyes, even
when intently examining it.1

A native species of spider (Thomisus citreus) exhibits a
somewhat similar alluring protection by its close resemblance
to buds of the wayfaring tree, Viburnum lantana. It is pure
creamy-white, the abdomen exactly resembling in shape and
colour the unopened buds of the flowers among which it takes
1 A Naturalist' s Wanderings in the Eastern Archipelago, p. 63.

212 DARWINISM

its station ■ and it has been seen to capture flies which came
to the flowers.

But the most curious and beautiful case of alluring protec-
tion is that of a wingless Mantis in India, which is so formed
and coloured as to resemble a pink orchis or some other
fantastic flower. -The whole insect is of a bright pink colour,
the large and oval abdomen looking like the labellum of
an orchid. On each side, the two posterior legs have im-
mensely dilated and flattened thighs which represent the
petals of a flower, while the neck and forelegs imitate the
upper sepal and column of an orchid. The insect rests
motionless, in this symmetrical attitude, among bright green
foliage, being of course very conspicuous, but so exactly
resembling a flower that butterflies and other insects settle
upon it and are instantly captured. It is a living trap,
baited in the most alluring manner to catch the unwary
flower-haunting insects.1

The Coloration of Birds' Eggs.

The colours of birds' eggs have long been a difficulty on
the theory of adaptive coloration, because, in so many cases
it has not been easy to see what can be the use of the par-
ticular colours, which are often so bright and conspicuous that
they seem intended to attract attention rather than to be con-
cealed. A more careful consideration of the subject in all its
bearings shows, however, that here too, in a great number of
cases, we have examples of protective coloration. When,
therefore, we cannot see the meaning of the colour, we may
suppose that it has been protective in some ancestral form,
and, not being hurtful, has persisted under changed condi-
tions which rendered the protection needless.

We may divide all eggs, for our present purpose, into two

1 A beautiful drawing of this rare iusect, Hymenopus bicornis (in the
nymph or active pupa state), was kindly sent me by Mr. Wood-Mason, Curator
of the Indian Museum at Calcutta. A species, very similar to it, inhabits Java,
where it is said to resemble a pink orchid. Other Mantidre, of the genus
Gongylus, have the anterior part of the thorax dilated and coloured either
white, pink, or purple ; and they so closely resemble flowers that, according
to Mr. Wood -Mason, one of them, having a bright violet-blue pro thoracic
shield, was found in Pegu by a botanist, and was for a moment mistaken by
him. for a flower. See Proc. Ent. Soc. Lond., 1878, p. liii.

via ORIGIN" AND USES OF COLOUR IN ANIMALS 213

great divisions ; those which are white or nearly so, and those
which are distinctly coloured or spotted. Egg-shells being com-
posed mainly of carbonate of lime, we may assume that the
primitive colour of birds' eggs was white, a colour that pre-
vails now among the other egg-bearing vertebrates — lizards,
crocodiles, turtles, and snakes ; and we might, therefore, expect
that this colour would continue where its presence had no
disadvantages. Now, as a matter of fact, we find that in all
the groups of birds which lay their eggs in concealed places,
whether in holes of trees or in the ground, or in domed or
covered nests, the eggs are either pure white or of very pale
uniform coloration. Such is the case with kingfishers, bee-
eaters, penguins, and puffins, which nest in holes in the
ground ; with the great parrot family, the woodpeckers, the
rollers, hoopoes, trogons, owls, and some others, which build in
holes in trees or other concealed places ; while martins, wrens,
willow- warblers, and Australian finches, build domed or covered
nests, and usually have white eggs.

There are, however, many other birds which lay their
white eggs in open nests ; and these afford some very in-
teresting examples of the varied modes by which concealment
may be obtained. All the duck tribe, the grebes, and the
pheasants belong to this class ; but these birds all have the
habit of covering their eggs with dead leaves or other material
whenever they leave the nest, so as effectually to conceal
them. Other birds, as the short-eared owl, the goatsucker,
the partridge, and some of the Australian ground pigeons,
lay their white or pale eggs on the bare soil ; but in these
cases the birds themselves are protectively coloured, so that,
when sitting, they are almost invisible; and they have the
habit of sitting close and almost continuously, thus effectually
concealing their eggs.

Pigeons and doves offer a very curious case of the protec-
tion of exposed eggs. They usually build very slight and
loose nests of sticks and twigs, so open that light can be
seen through them from below, while they are generally well
concealed by foliage above. Their eggs are white and
shining ; yet it is a difficult matter to discover, from beneath,
whether there are eggs in the nest or not, while they are well
hidden by the thick foliage above. The Australian podargi — ■

214 DARWINISM

huge goatsuckers — build very similar nests, and their white
eggs are protected in the same manner. Some large and
powerful birds, as the swans, herons, pelicans, cormorants, and
storks, lay white eggs in open nests ; but they keep careful
watch over them, and are able to drive away intruders. On
the whole, then, we see that, while white eggs are conspicuous,
and therefore especially liable to attack by egg-eating animals,
they are concealed from observation in many and various ways.
We may, therefore, assume that, in cases where there seems
to be no such concealment, we are too ignorant of the whole
of the conditions to form a correct judgment.

We now come to the large class of coloured or richly
spotted eggs, and here we have a more difficult task, though
many of them decidedly exhibit protective tints or markings.
There are two birds which nest on sandy shores — the lesser
tern and the ringed plover, — and both lay sand-coloured eggs,
the former spotted so as to harmonise with coarse shingle, the
latter minutely speckled like fine sand, which are the kinds
of ground the two birds choose respectively for their nests.
"The common sandpipers' eggs assimilate so closely with
the tints around them as to make their discovery a matter
of no small difficulty, as every oologist can testify who has
searched for them. The pewits' eggs, dark in ground
colour and boldly marked, are in strict harmony with the
sober tints of moor and fallow, and on this circumstance
alone their concealment and safety depend. The divers'
eggs furnish another example of protective colour ; they
are generally laid close to the water's edge, amongst drift
and shingle, where their dark tints and black spots conceal
them by harmonising closely with surrounding objects. The
snipes and the great army of sandpipers furnish innumer-
able instances of protectively coloured eggs. In all the
instances given the sitting - bird invariably leaves the eggs
uncovered when it quits them, and consequently their safety
depends solely on the colours which adorn them."1 The
wonderful range of colour and marking in the eggs of the
guillemot may be imputed to the inaccessible rocks on which

1 C. Dixon, in Seebohm's History of British Birds, vol. ii. Introduction, p.
xxvi. Many of the other examples here cited are taken from the same valu-
able work.

vixi ORIGIN AND USES OF COLOUR IN ANIMALS 215

it breeds, giving it complete protection from enemies. Thus
the pale or bluish ground colour of the eggs of its allies, the
auks and puffins, has become intensified and blotched and
spotted in the most marvellous variety of patterns, owing to
there being no selective agency to prevent individual variation
having full sway.

The common black coot (Fulica atra) has eggs which are
coloured in a specially protective manner. Dr. William
Marshall writes, that it only breeds in certain localities where
a large water reed (Phragmites arundinacea) abounds. The
eggs of the coot are stained and spotted with black on a
yellowish-gray ground, and the dead leaves of the reed are of
the same colour, and are stained black by small parasitic fungi
of the Uredo family ; and these leaves form the bed on Avhich
the eggs are laid. The eggs and the leaves agree so closely
in colour and markings that it is a difficult thing to dis-
tinguish-the eggs at any distance. It is to be noted that
the coot never covers up its eggs, as its ally the moor-hen
usually does.

The beautiful blue or greenish eggs of the hedge-sparrow,
the song-thrush, and sometimes those of the blackbird, seem at
first sight especially calculated to attract attention, but it is
very doubtful whether they are really so conspicuous when
seen at a little distance among their usual surroundings. For
the nests of these birds are either in evergreens, as holly or
ivy, or surrounded by the delicate green tints of our early
spring vegetation, and may thus harmonise very well with the
colours around them. The great majority of the eggs of our
smaller birds are so spotted or streaked with brown or black
on variously tinted grounds that, when lying in the shadow of
the nest and surrounded by the many colours and tints of
bark and moss, of purple buds and tender green or yellow
foliage, with all the complex glittering lights and mottled
shades produced among these by the spring sunshine and by
sparkling raindrops, they must have a quite different aspect
from that which they possess when we observe them torn
from their natural surroundings. We have here, probably,
a similar case of general protective harmony to that of the
green caterpillars with beautiful white or purple bands and
spots, which, though gaudily conspicuous when seen alone,

216 DARWINISM

become practically invisible among the complex lights and
shadows of the foliage they feed upon.

In the case of the cuckoo, which lays its eggs in the nests
of a variety of other birds, the eggs themselves are subject
to considerable variations of colour, the most common type,
however, resembling those of the pipits, wagtails, or warblers,
in whose nests they are most frequently laid. It also often
lays in the nest of the hedge-sparrow, whose bright blue eggs
are usually not at all nearly matched, although they are
sometimes said to be so on the Continent. It is the opinion
of many ornithologists that each female cuckoo lays the same
coloured eggs, and that it usually chooses a nest the owners
of Avhich lay somewhat similar eggs, though this is by no
means universally the case. Although birds which have
cuckoos' eggs imposed upon them do not seem to neglect them
on account of any difference of colour, yet they probably do
so occasionally ; and if, as seems probable, each bird's eggs are
to some extent protected by their harmony of colour with their
surroundings, the presence of a larger and very differently
coloured egg in the nest might be dangerous, and lead to the
destruction of the whole set. Those cuckoos, therefore, which
most frequently placed their eggs among the kinds which they
resembled, would in the long run leave most progeny, and
thus the very frequent accord in colour might have been
brought about.

Some writers have suggested that the varied colours of
birds' eggs are primarily due to the effect of surrounding-
coloured objects on the female bird during the period pre-
ceding incubation; and have expended much ingenuity in
suggesting the objects that may have caused the eggs of one
bird to be blue, another brown, and another pink.1 But no
evidence has been presented to prove that any effects what-
ever are produced by this cause, while there seems no difficulty
in accounting for the facts by individual variability and the
action of natural selection. The changes that occur in the
conditions of existence of birds must sometimes render the
concealment less perfect than it may once have been ; and
when any danger arises from this cause, it may be met either

1 See A. H. S. Lucas, in Proceedings of Royal Society of Victoria, 1887,
p. 56.

vin ORIGIN AND USES OF COLOUR IN ANIMALS 217

by some change in the colour of the eggs, or in the structure
or position of the nest, or by the increased care which the
parents bestow upon the eggs. In this way the various
divergences which now so often puzzle us may have arisen.

Colour as a Means of Recognition.

If we consider the habits and life-histories of those animals
which are more or less gregarious, comprising a large pro-
portion of the herbivora, some carnivora, and a considerable
number of all orders of birds, we shall see that a means of
ready recognition of its own kind, at a distance or during
rapid motion, in the dusk of twilight or in partial cover,
must be of the greatest advantage and often lead to the pre-
servation of life. Animals of this kind will not usually
receive a stranger into their midst. While they keep together
they are generally safe from attack, but a solitary straggler
becomes 'an easy prey to the enemy ; it is, therefore, of the
highest importance that, in such a case, the wanderer should
have every facility for discovering its companions with cer-
tainty at any distance within the range of vision.

Some means of easy recognition must be of vital im-
portance to the young and inexperienced of each flock, and it
also enables the sexes to recognise their kind and thus avoid
the evils of infertile crosses ; and I am inclined to believe that
its necessity has had a more widespread influence in deter-
mining the diversities of animal coloration than any other
cause whatever. To it may probably be imputed the singular
fact that, whereas bilateral symmetry of coloration is very
frequently lost among domesticated animals, it almost uni-
versally prevails in a state of nature ; for if the two sides of
an animal were unlike, and the diversity of coloration among
domestic animals occurred in a wild state, easy recognition
would be impossible among numerous closely allied forms.1

1 Professor Win. H. Brewer of Yale College lias shown that the white
marks or the spots of domesticated animals are rarely symmetrical, but have
a tendency to appear more frequently on the left side. This is the case with
horses, cattle, dogs, and swine. Among wild animals the skunk varies con-
siderably in the amount of white on the body, and this too was found to bo
usually greatest on the left side. A close examination of numerous striped or
spotted species, as tigers, leopards, jaguars, zebras, etc., showed that the
bilateral symmetry was not exact, although the general effect of the two sides

218 DARWINISM

The wonderful diversity of colour and of marking that pre-
vails, especially in birds and insects, may be due to the fact
that one of the first needs of a new species would be, to keep
separate from its nearest allies, and this could be most readily
done by some easily seen external mark of difference. A few
illustrations will serve to show how this principle acts in nature.

My attention was first called to the subject by a remark
of Mr. Darwin's that, though, " the hare on her form is a
familiar instance of concealment through colour, yet the
principle partly fails in a closely allied species, the rabbit ; for
when running to its burrow it is made conspicuous to the
sportsman, and no doubt to all beasts of prey, by its upturned
white tail."1 But a little consideration of the habits of the
animal will show that the white upturned tail is of the greatest
value, and is really, as it has been termed by a Avriter in The
Field, a "signal flag of danger." For the rabbit is usually a
crepuscular animal, feeding soon after sunset or on moonlight
nights. When disturbed or alarmed it makes for its burrow,
and the white upturned tails of those in front serve as guides
and signals to those more remote from home, to the young and
the feeble ; and thus each f ollowing the one or two before it, all
are able with the least possible delay to regain a place of
comparative safety. The apparent danger, therefore, becomes
a most important means of security.

The same general principle enables us to understand the
singular, and often conspicuous, markings on so many gregarious
herbivora which are yet, on the whole, protectively coloured.
Thus, the American prong -buck has a white patch behind
and a black muzzle. The Tartarian antelope, the Ovis poli
of High Asia, the Java wild ox, several species of deer, and a
large number of antelopes have a similar conspicuous white
patch behind, which, in contrast to the dusky body, must enable
them to be seen and followed from a distance by their fellows.
Where there are many species of nearly the same general size
and form inhabiting the same region — as with the antelopes

was the same. This is precisely what we should expect if the symmetry is not
the result of a general law of the organisation, but has been, in part at least, pro-
duced and preserved for the useful jiurpose of recognition by the animal's
fellows of the same species, and especially by the sexes and the young. See
Proc. of the Am. Ass. for Advancement of Science, vol. xxx. p. 246.
1 Descent of Man, p. 542.

viii ORIGIN AND USES OF COLOUR IN ANIMALS 219

of Africa — we find many distinctive markings of a similar
kind. The gazelles have variously striped and banded faces,
besides white patches behind and on the flanks, as shown
in the woodcut. The spring-bok has a white patch on the
face and one on the sides, with a curiously distinctive white
stripe above the tail, which is nearly concealed when the
animal is at rest by a fold of skin but comes into full view
when it is in motion, being thus quite analogous to the

Fig. 18.— Gazella scemmerringi.

upturned white tail of the rabbit. In the pallah the
white rump-mark is bordered with black, and the peculiar
shape of the horns distinguishes it when seen from the
front. The sable-antelope, the gems-bok, the oryx, the hart-
beest, the bonte-bok, and the addax have each peculiar white
markings ; and they are besides characterised by horns so
remarkably different in each species and so conspicuous, that
it seems probable that the peculiarities in length, twist, and
curvature have been differentiated for the purpose of recogni-
tion, rather than for any speciality of defence in species whose
general habits are so similar.

220 DARWINISM

It is interesting to note that these markings for recognition
are very slightly developed in the antelopes of the woods and
marshes. Thus, the grys-bok is nearly uniform in colour, except
the long black-tipped ears ; and it frequents the wooded moun-
tains. The duyker-bok and the rhoode-bok are wary bush-
haunters, and have no marks but the small white patch
behind. The wood-haunting bosch-bok goes in pairs, and has
hardly any distinctive marks on its dusky chestnut coat, but
the male alone is horned. The large and handsome koodoo
frequents brushwood, and its vertical white stripes are no
doubt protective, while its magnificent spiral horns afford easy
recognition. The eland, which is an inhabitant of the open
country, is uniformly coloured, being sufficiently recognisable
by its large size and distinctive form ; but the Derbyan eland
is a forest animal, and has a protectively striped coat. In like
manner, the fine Speke's antelope, which lives entirely in the
swamps and among reeds, has pale vertical stripes on the
sides (protective), with white markings on face and breast for
recognition. An inspection of the figures of antelopes and
other animals in Wood's Natural History, or in other illustrated
works, will give a better idea of the peculiarities of recognition
markings than any amount of description.

Other examples of such coloration are to be seen in the
dusky tints of the musk-sheep and the reindeer, to whom
recognition at a distance on the snowy plains is of more
importance than concealment from their few enemies. The
conspicuous stripes and bands of the zebra and the quagga are
probably due to the same cause, as may be the singular crests
and face-marks of several of the monkeys and lemurs.1

1 It may be thought that such extremely conspicuous markings as those of
the zebra would be a great danger in a country abounding with lions, leopards,
and other beasts of prey ; but it is not so. Zebras usually go in bands, and
are so swift and wary that they are in little danger during the day. It is in
the evening, or on moonlight nights, when they go to drink, that they are chiefly
exposed to attack ; and Mr. Francis Galton, who has studied these animals in their
native haunts, assures me, that in twilight they are not at all conspicuous,
the stripes of white and black so merging together into a gray tint that it is
very difficult to see them at a little distance. We have here an admirable
illustration of how a glaringly conspicuous style of marking for recognition may
be so arranged as to become also jirotective at the time when protection is
most needed ; and we may also learn how impossible it is for us to decide
on the inutility of any kind of coloration without a careful study of tlie
habits of the species in its native country.

ORIGIN AND USES OF COLOUR IN ANIMALS

221

<s

o

U

ZJ

1

222 DARWINISM

Among birds, these recognition marks are especially
numerous and suggestive. Species which inhabit open
districts are usually protectively coloured ; but they generally
possess some distinctive markings for the purpose of being
easily recognised by their kind, both when at rest and during
flight. Such are, the white bands or patches on the breast
or belly of many birds, but more especially the head and
neck markings in the form of white or black caps, collars,
eye-marks or frontal patches, examples of which are seen in
the three species of African plovers figured on page 221.

Recognition marks during flight are very important for all
birds which congregate in flocks or which migrate together ;
and it is essential that, while being as conspicuous as possible,
the marks shall not interfere with the general protective tints
of the species when at rest. Hence they usually consist of
well -contrasted markings on the wings and tail, which are
concealed during repose but become fully visible when the
bird takes flight. Such markings are well seen in our four
British species of shrikes, each having quite different white
marks on the expanded wings and on the tail feathers ; and
the same is the case with our three species of Saxicola — the
stone-chat, whin-chat, and wheat-ear — which are thus easily
recognisable on the wing, especially when seen from above, as
they would be by stragglers looking out for their companions.
The figures opposite, of the wings of two African species
of stone curlew which are sometimes found in the same
districts, well illustrates these specific recognition marks.
Though not very greatly different to our eyes, they are no
doubt amply so to the sharp vision of the birds themselves.

Besides the white patches on the primaries here shown, the
secondary feathers are, in some cases, so coloured as to afford
very distinctive markings during flight, as seen in the central
secondary quills of two African coursers (Fig. 21).

Most characteristic of all, however, are the varied markings
of the outer tail-feathers, Avhose purpose is so well shown by
their being almost always covered during repose by the two
middle feathers, which are themselves quite unmarked and
protectively tinted like the rest of the upper surface of the
body. The figures of the expanded tails of two species of East
Asiatic snipe, whose geographical ranges overlap each other,

ORIGIN AND USES OF COLOUR IN ANIMALS

223

Bare " -

Fig. 20.— CEdicnemus veimiculatus (above). 05. sencgalensis (below).

224

DARWINISM

will serve to illustrate this difference ; which is frequently much
greater and modified in an endless variety of ways (Fig. 22).
Numbers of species of pigeons, hawks, finches, warblers,
ducks, and innumerable other birds possess this class of mark-
ings ; and they correspond so exactly in general character with

Cursorius chalcopterus. C. gallieus.

Fig. 21. — Secondary quills.

those of the mammalia, already described, that we cannot
doubt they serve a similar purpose.1

Those birds which are inhabitants of tropical forests, and
which need recognition marks that shall be at all times
visible among the dense foliage, and not solely or chiefly
during flight, have usually small but brilliant patches of colour

1 The principle of colouring for recognition was, I believe, first stated in
my article on " The Colours of Animals and Plants " in Macmillan's Magazine,
and more fully in my volume on Tropical Nature. Subsequently Mrs. Barber
gave a few examples under the head of " Indicative or Banner Colours," but
she applied it to the distinctive colours of the males of birds, which I explain
on another principle, though this may assist.

viii ORIGIN AND USES OF COLOUR IN ANIMALS 225

Fig. 22,— Scolopax megala (upper). S. stermra (lower).

Q

226 DARWINISM

on the head or neck, often not interfering with the generally
protective character of their plumage. Such are the bright
patches of blue, red, or yellow, by which the usually green
Eastern barbets are distinguished ; and similar bright patches
of colour characterise the separate species of small green
fruit-doves. To Jbhis necessity for specialisation in colour, by
which each bird may easily recognise its kind, is probably due
that marvellous variety in the peculiar beauties of some groups
of birds. The Duke of Argyll, speaking of the humming
birds, made the objection that "A crest of topaz is no
better in the struggle for existence than a crest of sapphire.
A frill ending in spangles of the emerald is no better in the
battle of life than a frill ending in spangles of the ruby. A
tail is not affected for the purposes of flight, whether its
marginal or its central feathers are decorated with white ;"
and he goes on to urge that mere beauty and variety for
their own sake are the only causes of these differences. But,
on the principles here suggested, the divergence itself is useful,
and must have been produced pari passu with the structural
differences on which the differentiation of species depends ;
and thus we have explained the curious fact that prominent
differences of colour often distinguish species otherwise very
closely allied to each other.

Among insects, the principle of distinctive coloration for
recognition has probably been at work in the production of
the wonderful diversity of colour and marking we find every-
where, more especially among the butterflies and moths ; and
here its chief function may have been to secure the pairing
together of individuals of the same species. In some of the
moths this has been secured by a peculiar odour, which
attracts the males to the females from a distance ; but there is
no evidence that this is universal or even general, and among
butterflies, especially, the characteristic colour and marking,
aided by size and form, afford the most probable means of
recognition. That this is so is shown by the fact that " the
common white butterfly often flies down to a bit of paper on
the ground, no doubt mistaking it for one of its own species ;"
while, according to Mr. Collingwood, in the Malay Archipelago,
" a dead butterfly pinned upon a conspicuous twig will often
arrest an insect of the same species in its headlong flight, and

vni ORIGIN AND USES OF COLOUR IN ANIMALS 227

bring it down within easy reach of the net, especially if it be
of the opposite sex."1 In a great number of insects, no doubt,
form, motions, stridulating sounds, or peculiar odours, serve to
distinguish allied species from each other, and this must be
especially the case with nocturnal insects, or with those whose
colours are nearly uniform and are determined by the need of
protection ; but by far the larger number of day-flying and
active insects exhibit varieties of colour and marking, forming
the most obvious distinction between allied species, and which
have, therefore, in all probability been acquired in the process
of differentiation for the purpose of checking the intercrossing
of closely allied forms.2

Whether this principle extends to any of the less highly
organised animals is doubtful, though it may perhaps have
affected the higher mollusca. But in marine animals it seems
probable that the colours, however beautiful, varied, and
brilliant- they may often be, are in most cases protective,
assimilating them to the various bright-coloured seaweeds, or
to some other animals which it is advantageous for them to
imitate.3

Summary of the Preceding Exposition.

Before proceeding to discuss some of the more recondite
phenomena of animal coloration, it will be well to consider
for a moment the extent of the ground we have already
covered. Protective coloration, in some of its varied forms,
has not improbably modified the appearance of one-half of
the animals living on the globe. The white of arctic animals,
the yellowish tints of the desert forms, the dusky hues of
crepuscular and nocturnal species, the transparent or bluish
tints of oceanic creatures, represent a vast host in themselves ;
but we have an equally numerous body whose tints are
adapted to tropical foliage, to the bark of trees, or to the soil

1 Quoted by Darwin in Descent of Man, p. 317.

2 In the American Naturalist of March 1888, Mr. J. E. Todd has an
article on " Directive Coloration in Animals," in which he recognises many of
the cases here referred to, and suggests a few others, though I think he
includes many forms of coloration — as " paleness of belly and inner side of
legs " — which do not belong to this class.

3 For numerous examples of this protective colouring of marine animals
see Moseley's Voyage of the Challenger, and Dr. E. S. Morse in Proc. ofBost.
Soc. of Nat. Hist., vol. xiv. 1871.

228 DARWINISM

or dead leaves on or among which they habitually live. Then
we have the innumerable special adaptations to the tints and
forms of leaves, or twigs, or flowers ; to bark or moss ; to rock
or pebble ; by which such vast numbers of the insect tribes
obtain protection ; and we have seen that these various forms
of coloration are .equally prevalent in the waters of the seas
and oceans, and are thus coextensive with the domain of life
upon the earth. The comparatively small numbers which
possess "terrifying" or "alluring" coloration may be classed
under the general head of the protectively coloured.

But under the next head — colour for recognition — we have
a totally distinct category, to some extent antagonistic or
complementary to the last, since its essential principle is
visibility rather than concealment. Yet it has been shown, I
think, that this mode of coloration is almost equally im-
portant, since it not only aids in the preservation of existing
species and in the perpetuation of pure races, but was, per-
haps, in its earlier stages, a not unimportant factor in their
development. To it we owe most of the variety and much
of the beauty in the colours of animals ; it has caused at
once bilateral symmetry and general permanence of type ;
and its range of action has been perhaps equally extensive
with that of coloration for concealment.

Influence of Locality or of Climate on Colour.

Certain relations between locality and coloration have long
been noticed. Mr. Gould observed that birds from inland or
continental localities were more brightly coloured than those
living near the sea-coast or on islands, and he supposed that
the more brilliant atmosphere of the inland stations was the
explanation of the phenomenon.1 Many American naturalists
have observed similar facts, and they assert that the intensity
of the colours of birds and mammals increases from north to
south, and also with the increase of humidity. This change
is imputed by Mr. J. A. Allen to the direct action of the en-
vironment. He says : "In respect to the correlation of intensity
of colour in animals with the degree of humidity, it would
perhaps be more in accordance with cause and effect to express
the law of correlation as a decrease of intensity of colour with
1 See Origin of Species, -p. 107.

vin ORIGIN AND USES OF COLOUR IN ANIMALS 229

a decrease of humidity, the paleness evidently resulting from
exposure and the blanching effect of intense sunlight, and a
dry, often intensely heated atmosphere. With the decrease of
the aqueous precipitation the forest growth and the protection
afforded by arborescent vegetation gradually also decreases, as
of course does also the protection afforded by clouds, the
excessively humid regions being also regions of extreme
cloudiness, while the dry regions are comparatively cloudless
districts." * Almost identical changes occur in birds, and are
imputed by Mr. Allen to similar causes.

It will be seen that Mr. Gould and Mr. Allen impute
opposite effects to the same cause, brilliancy or intensity of
colour being due to a brilliant atmosphere according to the
former, while paleness of colour is imputed by the latter to
a too brilliant sun. According to the principles which have
been established by the consideration of arctic, desert, and
forest animals respectively, we shall be led to conclude that
there has been no direct action in this case, but that the effects
observed are due to the greater or less need of protection.
The pale colour that is prevalent in arid districts is in harmony
with the general tints of the surface ; while the brighter tints
or more intense coloration, both southward and in humid
districts, are sufficiently explained by the greater shelter due
to a more luxuriant vegetation and a shorter winter. The
advocates of the theory that intensity of light directly affects
the colours of organisms, are led into perpetual inconsistencies.
At one time the brilliant colours of tropical birds and insects
are imputed to the intensity of a tropical sun, while the same
intensity of sunlight is now said to have a " bleaching " effect.
The comparatively dull and sober hues of our northern fauna
were once supposed to be the result of our cloudy skies ; but
now we are told that cloudy skies and a humid atmosphere
intensify colour.

In my Tropical Nature (pp. 257-264) I have called atten-
tion to what is perhaps the most curious and decided relation
of colour to locality which has yet been observed — the preval-
ence of white markings in the butterflies and birds of islands.

1 The " Geographical Variation of North American Squirrels," Proc. Bost.
Soc. of Nat. Hist., 1874, p. 284; and Mammals and Winter Birds of Florida,^).
233-241.

230 DARWINISM

So many cases are adduced from so many different islands, both
in the eastern and western hemisphere, that it is impossible
to doubt the existence of some common cause ; and it seems
probable to me now, after a fuller consideration of the whole
subject of colour, that here too Ave have one of the almost innumer-
able results of the 'principle of protective coloration. White is,
as a rule, an uncommon colour in animals, but probably only
because it is so conspicuous. Whenever it becomes pro-
tective, as in the case of arctic animals and aquatic birds, it
appears freely enough ; while we know that white varieties
of many species occur occasionally in the wild state, and
that, under domestication, white or parti-colourecl breeds are
freely produced. Now in all the islands in which exception-
ally white-marked birds and butterflies have been observed,
we find two features which would tend to render the con-
spicuous white markings less injurious — a luxuriant tropical
vegetation, and a decided scarcity of rapacious mammals and
birds. White colours, therefore, would not be eliminated
by natural selection ; but variations in this direction would
bear their part in producing the recognition marks which
are everywhere essential, and which, in these islands, need
not be so small or so inconspicuous as elsewhere.

Concluding Remarks.

On a review of the whole subject, then, we must conclude
that there is no evidence of the individual or prevalent colours
of organisms being directly determined by the amount of light,
or heat, or moisture, to which they are exposed ; while, on the
other hand, the two great principles of the need of concealment
from enemies or from their prey, and of recognition by their
own kind, are so wide-reaching in their application that they
appear at first sight to cover almost the whole ground of
animal coloration. But, although they are indeed wonderfully
general and have as yet been very imperfectly studied, we are
acquainted with other modes of coloration which have a
different origin. These chiefly appertain to the very singular class
of warning colours, from which arise the yet more extraordinary
phenomena of mimicry ; and they open up so curious a field
of inquiry and present so many interesting problems, that a
chapter must be devoted to them. Yet another chapter will

vin ORIGIN AND USES OF COLOUR IN ANIMALS 231

be required by the subject of sexual differentiation of colour
and ornament, as to the origin and meaning of which I have
arrived at different conclusions from Mr. Darwin. These vari-
ous forms of coloration having been discussed and illustrated,
we shall be in a position to attempt a brief sketch of the funda-
mental laws which have determined the general coloration of
the animal world.

CHAPTER IX

WARNING COLORATION AND MIMICRY

The skunk as an example of warning coloration — "Warning colours among
insects — Butterflies — Caterpillars — Mimicry — How mimicry has been
produced — Heliconidse — Perfection of the imitation — Other cases of
mimicry among Lepidoptera — Mimicry among protected groups — Its
explanation — Extension of the principle — Mimicry in other orders
of insects — Mimicry among the vertebrata — Snakes — The rattlesnake
and the cobra — Mimicry among birds — Objections to the theory of
mimicry — Concluding remarks on warning colours and mimicry.

We have now to deal with a class of colours which are
the very opposite of those we have hitherto considered, since,
instead of serving to conceal the animals that possess them
or as recognition marks to their associates, they are developed
for the express purpose of rendering the species conspicuous.
The reason of this is that the animals in question are either
the possessors of some deadly weapons, as stings or poison
fangs, or they are uneatable, and are thus so disagree-
able to the usual enemies of their kind that they are never
attacked when their peculiar powers or properties are known.
It is, therefore, important that they should not be mis-
taken for defenceless or eatable species of the same class or
order, since in that case they might suffer injury, or even death,
before their enemies discovered the danger or the uselessness
of the attack. They require some signal or danger -flag
which shall serve as a warning to would-be enemies not to
attack them, and they have usually obtained this in the
form of conspicuous or brilliant coloration, very distinct
from the protective tints of the defenceless animals allied to
them.

chap, ix WARNING COLORATION AND MIMICRY 233

The Skunk as illustrating Warning Coloration.

While staying a few days, in July 1887, at the Summit
Hotel on the Central Pacific Railway, I strolled out one evening
after dinner, and on the road, not fifty yards from the house,
I saw a pretty little white and black animal with a bushy tail
coming towards me. As it came on at a slow pace and with-
out any fear, although it evidently saw me, I thought at first
that it must be some tame creature, when it suddenly occurred
to me that it was a skunk. It came on till within five or six
yards of me, then quietly climbed over a dwarf wall and dis-
appeared under a small outhouse, in search of chickens, as the
landlord afterwards told me. This animal possesses, as is well
known, a most offensive secretion, which it has the power of
ejecting over its enemies, and which effectually protects it
from attack. The odour of this substance is so penetrating
that it taints, and renders useless, everything it touches,
or in its vicinity. Provisions near it become uneatable, and
clothes saturated with it will retain the smell for several
weeks, even though they are repeatedly washed and dried.
A drop of the liquid in the eyes will cause blindness, and
Indians are said not unfrequently to lose their sight from this
cause. Owing to this remarkable power of offence the skunk
is rarely attacked by other animals, and its black and white
fur, and the bushy white tail carried erect when disturbed,
form the danger-signals by which it is easily distinguished in
the twilight or moonlight from unprotected animals. Its
consciousness that it needs only to be seen to be avoided gives
it that slowness of motion and fearlessness of aspect which
are, as we shall see, characteristic of most creatures so pro-
tected.

Warning Colours among Insects.

It is among insects that warning colours are best developed,
and most abundant. We all know how well marked and
conspicuous are the colours and forms of the stinging wasps
and bees, no one of which in any part of the world is known
to be protectively coloured like the majority of defenceless
insects. Most of the great tribe of Malacoderms among
beetles are distasteful to insect-eatine; animals. Our red and

234 DARWINISM

black Telephoridse, commonly called "soldiers and sailors,"
were found, by Mr. Jenner Weir, to be refused by small
birds. These and the allied Lampyridae (the fire -flies and
glow-worms) in Nicaragua, were rejected by Mr. Belt's tame
monkey and by his fowls, though most other insects were
greedily eaten by -them. The Coccinellidse or lady-birds are
another uneatable group, and their conspicuous and singularly
spotted bodies serve to distinguish them at a glance from all
other beetles.

These uneatable insects are probably more numerous than
is supposed, although we already know immense numbers
that are so protected. The most remarkable are the three
families of butterflies — Heliconida?, Danaidse, and Acrasidae —
comprising more than a thousand species, and characteristic re-
spectively of the three great tropical regions — South America,
Southern Asia, and Africa. All these butterflies have
peculiarities which serve to distinguish them from every
other group in their respective regions. They all have ample
but rather • weak wings, and fly slowly ; they are always very
abundant ; and they all have conspicuous colours or markings,
so distinct from those of other families that, in conjunction
with their peculiar outline and mode of flight, they can
usually be recognised at a glance. Other distinctive features
are, that their colours are always nearly the same on the
under surface of their wings as on the upper ; they never try
to conceal themselves, but rest on the upper surfaces of
leaves or flowers ; and, lastly, they all have juices which
exhale a poAverful scent, so that when one kills them by
pinching the body, the liquid that exudes stains the fingers
yellow, and leaves an odour that can only be removed by
repeated washings.

Now, there is much direct evidence to show that this
odour, though not very offensive to us, is so to most insect-
eating creatures. Mr. Bates observed that, when set out to
dry, specimens of Heliconidse were less subject to the attacks
of vermin ; while both he and I noticed that they were not
attacked by insect-eating birds or dragonflies, and that their
wings were not found in the forest paths among the numerous
wings of other butterflies Avhose bodies had been devoured.
Mr. Belt once observed a pair of birds capturing insects for

WARNING COLORATION AND MIMICRY

their young ; and although the Heliconidee swarmed in the
vicinity, and from their slow flight could have been easily
caught, not one was ever pursued, although other butterflies
did not escape. His tame monkey also, which would greedily
munch up other butterflies, would never eat the Heliconidae.
It would sometimes smell them, but always rolled them up in
its hand and then dropped them.

We have also some corresponding evidence as to the
distastefulness of the Eastern Danaidee. The Hon. Mr.
Justice Newton, who assiduously collected and took notes
upon the Lepidoptera of Bombay, informed Mr. Butler of the
British Museum that the large and swift -flying butterfly
Charaxes psaphon, was continually persecuted by the bulbul,
so that he rarely caught a specimen of this species which had
not a piece snipped out of the hind wings. He offered one to
a bulbul which he had in a cage, and it was greedily devoured,
whilst it was only by repeated persecution that he succeeded
in inducing the bird to touch a Danais.1

Besides these three families of butterflies, there are certain
groups of the great genus Papilio — the true swallow -tailed
butterflies — which have all the characteristics of uneatable
insects. They have a special coloration, usually red and
black (at least in the females), they fly slowly, they are very
abundant, and they possess a peculiar odour somewhat like
that of the Heliconidse. One of these groups is common in
tropical America, another in tropical Asia, and it is curious
that, although not very closely allied, they have each the same
red and black colours, and are very distinct from all the other
butterflies of their respective countries. There is reason to
believe also that many of the brilliantly coloured and weak-
flying diurnal moths, like the fine tropical Agaristidae and
bur net- moths, are similarly protected, and that their con-
spicuous colours serve as a warning of inedibility. The
common burnet-moth (Anthrocera filipendula) and the equally
conspicuous ragwort-moth (Euchelia Jacobean) have been proved
to be distasteful to insect-eating creatures.

1 Nature, vol. iii. p. 165. Professor Meldola observed that specimens of
Danais and Euplsea in collections were less subject to the attacks of mites
(Proc. Ent. Soc, 1877, p. xii.) ; and this was corroborated by Mr. Jenner Weir.
Entomologist, 1882, vol. xv. p. 160.

236 DARWINISM

The most interesting and most conclusive example of
warning coloration is, however, furnished by caterpillars,
because in this case the facts have been carefully ascertained
experimentally by competent observers. In the year 1866,
when Mr. Darwin was collecting evidence as to the supposed
effect of sexual selection in bringing about the brilliant
coloration of the higher animals, he was struck by the fact
that many caterpillars have brilliant and conspicuous colours,
in the production of which sexual selection could have no
place. We have numbers of such caterpillars in this country,
and they are characterised not only by their gay colours but
by not concealing themselves. Such are the mullein and the
gooseberry caterpillars, the larvae of the spurge hawk-moth, of
the buff-tip, and many others. Some of these caterpillars are
wonderfully conspicuous, as in the case of that noticed by
Mr. Bates in South America, which was four inches long,
banded across with black and yellow, and with bright red
head, legs, and tail. Hence it caught the eye of any one who
passed by, even at the distance of many yards.

Mr. Darwin asked me to try and suggest some explanation
of this coloration ; and, having been recently interested in
the question of the warning coloration of butterflies, I
suggested that this was probably a similar case, — that these
conspicuous caterpillars were distasteful to birds and other
insect-eating creatures, and that their bright non- protective
colours and habit of exposing themselves to view, enabled
their enemies to distinguish them at a glance from the edible
kinds and thus learn not to touch them; for it must be
remembered that the bodies of caterpillars while growing-
are so delicate, that a wound from a bird's beak would be
perhaps as fatal as if they were devoured.1 At this time not
a single experiment or observation had been made on the
subject, but after I had brought the matter before the
Entomological Society, two gentlemen, who kept birds and
other tame animals, undertook to make experiments with a
variety of cater pillars.

Mr. Jenner Weir was the first to experiment with ten
species of small birds in his aviary, and he found that none of
them would eat the following smooth-skinned conspicuous cater-
1 See Darwin's Descent of Man, p. 325.

ix WARNING COLORATION AND MIMICRY 237

pillars — Abraxas grossulariata, Diloba caeraleocephala, An-
throcera filipendula, and Cucullia verbasci. He also found that
they would not touch any hairy or spiny larvae, and he was
satisfied that it was not the hairs or the spines, but the un-
pleasant taste that caused them to be rejected, because in one
case a young smooth larva of a hairy species, and in another
case the pupa of a spiny larva, were equally rejected. On
the other hand, all green or brown caterpillars as well as
those that resemble twigs were greedily devoured.1

Mr. A. G. Butler also made experiments with some green
lizards (Lacerta viridis), which greedily ate all kinds of food,
including flies of many kinds, spiders, bees, butterflies, and
green caterpillars ; but they would not touch the caterpillar of
the gooseberry-moth (Abraxas grossulariata), or the imago of
the burnet-moth (Anthrocera filipendula). The same thing
happened with frogs. When the gooseberry caterpillars
were first given to them, "they sprang forward and licked
them eagerly into their mouths ; no sooner, however, had
they done so, than they seemed to become aware of the
mistake that they had made, and sat with gaping mouths,
rolling their tongues about, until they had got quit of the
nauseous morsels, which seemed perfectly uninjured, and
walked off as briskly as ever." Spiders seemed equally to
dislike them. This and another conspicuous caterpillar
(Halia wavaria) were rejected by two species — the geometrical
garden spider (Epeira diadema) and a hunting spider.2

Some further experiments with lizards were made by
Professor Weismann, quite confirming the previous observa-
tions; and in 1886 Mr. E. B. Poulton of Oxford undertook
a considerable series of experiments, with many other species of
larvae and fresh kinds of lizards and frogs. Mr. Poulton then
reviewed the whole subject, incorporating all recorded facts, as
well as some additional observations made by Mr. Jenner Weir
in 1886. More than a hundred species of larvae or of perfect
insects of various orders have now been made the subject of
experiment, and the results completely confirm my original
suggestion. In almost every case the protectively coloured
larvae have been greedily eaten by all kinds of insectivorous

1 Transactions of the Entomological Society of London, 1869, p. 21.
2 Ibid., p. 27.

238 DARWINISM

animals, "while, in the immense majority of cases, the con-
spicuous, hairy, or brightly coloured larva? have been rejected
by some or all of them. In some instances the inedibility of
the larvae extends to the perfect insect, but not in others. In
the former cases the perfect insect is usually adorned with
conspicuous colours, as the burnet and ragwort moths ; but
in the case of the buff-tip, the moth resembles a broken piece
of rotten stick, yet it is partly inedible, being refused by
lizards. It is, however, very doubtful whether these are its
chief enemies, and its protective form and colour may be
needed against insectivorous birds or mammals.

Mr. Samuel H. Scudder, who has largely bred North
American butterflies, has found so many of the eggs and larva?
destroyed by hymenopterous and dipterous parasites that he
thinks at least nine-tenths, perhaps a greater proportion, never
reach maturity. Yet he has never found any evidence that
such parasites attack either the egg or the larva of the inedible
Danais archippus, so that in this case the insect is distasteful
to its most dangerous foes in all the stages of its existence,
a fact which serves to explain its great abundance and its
extension over almost the whole world.1

One case has been found of a protectively coloured larva,
— one, moreover, which in all its habits shows that it
trusts to concealment to escape its enemies — which was yet
always rejected by lizards after they had seized it, evidently
under the impression that from its colour it would be
eatable. This is the caterpillar of the very common moth
Mania typica ; and Mr. Poulton thinks that, in this case, the
unpleasant taste is an incidental result of some physiological
processes in the organism, and is itself a merely useless
character. It is evident that the insect would not conceal
itself so carefully as it does if it had not some enemies, and
these are probably birds or small mammals, as its food-plants
are said to be dock and willow-herb, not suggestive of places
frequented by lizards ; and it has been found by experiment
that lizards and birds have not always the same likes and
dislikes. The case is interesting, because it shows that
nauseous fluids sometimes occur sporadically, and may thus be
intensified by natural selection when required for the purpose
1 Nature, vol. iii. p. 147.

ix WARNING COLORATION AND MIMICRY 239

of protection. Another exceptional case is that of the very
conspicuous caterpillar of the spurge hawk-moth (Deilephila
euphorbise), which was at once eaten by a lizard, although, as
it exposes itself on its food-plant in the daytime and is very
abundant in some localities, it must almost certainly he disliked
by birds or by some animals who would otherwise devour it.
If disturbed while feeding it is said to turn round with fury
and eject a quantity of green liquid, of an acid and disagree-
able smell similar to that of the spurge milk, only worse.1

These facts, and Mr. Poulton's evidence that some larvse
rejected by lizards at first will be eaten if the lizards are very
hungry, show that there are differences in the amount of the
distastefulness, and render it probable that if other food were
wanting many of these conspicuous insects would be eaten.
It is the abundance of the eatable kinds that gives value to
the inedibility of the smaller number ; and this is probably
the reason why so many insects rely on protective colouring
rather than on the acquisition of any kind of defensive
weapons. In the long run the powers of attack and defence
must balance each other. Hence we see that even the power-
ful stings of bees and wasps only protect them against some
enemies, since a tribe of birds, the bee -eaters, have been
developed which feed upon them, and some frogs and lizards
do so occasionally.

The preceding outline will sufficiently explain the character-
istics of " warning coloration " and the end it serves in nature.
There are many other curious modifications of it, but these will
be best appreciated after we have discussed the remarkable
phenomenon of "mimicry," which is bound up with and
altogether depends upon " warning colour," and is in some
cases the chief indication we have of the possession of some
offensive weapon to secure the safety of the species imitated.

Mimicry.

This term has been given to a form of protective resem-
blance, in which one species so closely resembles another in
external form and colouring as to be mistaken for it, although
the two may not be really allied and often belong to distinct

1 Stainton's Manual of Butterflies and Moths, vol. i. p. 93 ; E. B.
Poulton, Proceedings of the Zool. Soc. of London, 1887, pp. 191-274.

240 DARWINISM

families or orders. One creature seems disguised in order to
be made like another ; hence the terms "mimic" and mimicry,
which imply no voluntary action on the part of the imitator.
It has long been known that such resemblances do occur, as, for
example, the clear-winged moths of the families Sesiidae and
iEgeriidse, many of which resemble bees, wasps, ichneumons,
or saw-flies, and have received names expressive of the re-
semblance ; and the parasitic flies (Volucella) which closely
resemble bees, on whose larvse the larvae of the flies feed.

The great bulk of such cases remained, however, unnoticed,
and the subject was looked upon as one of the inexplicable
curiosities of nature, till Mr. Bates studied the phenomenon
among the butterflies of the Amazon, and, on his return home,
gave the first rational explanation of it.1 The facts are, briefly,
these. Everywhere in that fertile region for the entomologist
the brilliantly coloured Heliconiclse abound, with all the char-
acteristics which I have already referred to when describing
them as illustrative of "warning coloration." But along
with them other butterflies were occasionally captured, which,
though often mistaken for them, on account of their close
resemblance in form, colour, and mode of flight, were found
on examination to belong to a very distinct family, the Pieridae.
Mr. Bates notices fifteen distinct species of Pieridae, belonging
to the genera Leptalis and Euterpe, each of which closely imitates
some one species of Heliconidae, inhabiting the same region and
frequenting the same localities. It must be remembered that
the two families are altogether distinct in structure. The
larvee of the Heliconidae are tubercled or spinecl, the pupae sus-
pended head downwards, and the imago has imperfect fore-
legs in the male ; while the larvae of the Pieridae are smooth,
the pupae are suspended with a brace to keep the head erect,
and the forefeet are fully developed in both sexes. These
differences are as large and as important as those between pigs
and sheep, or between swallows and sparrows ; while English
entomologists will best understand the case by supposing that
a species of Pieris in this country was coloured and shaped
like a small tortoise-shell, while another species on the Con-
tinent was equally like a Camberwell beauty — so like in both

1 See Transactions of the Linnean Society, vol. xxiii. pp. 495-566, coloured
plates.

WARNING COLORATION AND MIMICRY

241

cases as to be mistaken when on the wing, and the difference
only to be detected by close examination. As an example of
the resemblance, woodcuts are given of one pair in which the
colours are simple, being olive, yellow, and black, while the

Fia. 23.— Methona psidii (Heliconidoe). Leptalis orisc (Pieridse).

very distinct neuration of the wings and form of the head and
body can be easily seen.

Besides these Pieridse, Mr. Bates found four true Papilios,
seven Erycinidse, three Castnias (a genus of day-flying moths),
and fourteen species of diurnal Bombycidse, all imitating some
species of Heliconidse which inhabited the same district ; and
it is to be especially noted that none of these insects were so
•abundant as the Heliconidse they resembled, generally they

242 DARWINISM

were far less common, so that Mr. Bates estimated the pro-
portion in some cases as not one to a - thousand. Before
giving an account of the numerous remarkable cases of mimicry
in other parts of the world, and between various groups
of insects and of higher animals, it will be well to explain
briefly the use and purport of the phenomenon, and also the
mode by which it has been brought about.

Row Mimicry has been Produced.

The fact has been now established that the Heliconidae
possess an offensive odour and taste, which lead to their
being almost entirely free from attack by insectivorous
creatures ; .they possess a peculiar form and mode of flight,
and do not seek concealment ; while their colours — although
very varied, ranging from deep blue-black, with white, yellow,
or vivid red bands and spots, to the most delicate semitrans-
parent wings adorned with pale brown or yellow markings —
are yet always very distinctive, and unlike those of all the
other families of butterflies in the same country. It is,
therefore, clear that if any other butterflies in the same
region, which are eatable and suffer great persecution from
insectivorous animals, should come to resemble any of these
uneatable species so closely as to be mistaken for them by
their enemies, they Avill obtain thereby immunity from per-
secution. This is the obvious and sufficient reason why the
imitation is useful, and therefore why it occurs in nature. We
have now to explain how it has probably been brought about,
and also why a still larger number of persecuted groups have
not availed themselves of this simple means of protection.

From the great abundance of the Heliconidae1 all over
tropical America, the vast number of their genera and species,
and their marked distinctions from all other butterflies, it
follows that they constitute a group of high antiquity, which
in the course of ages has become more and more specialised,
and owing to its peculiar advantages has now become a
dominant and aggressive race. But when they first arose
from some ancestral species or group which, owing to the food

1 These butterflies are now divided into two sub-families, one of which is
placed with the Danaidse ; but to avoid confusion I shall always speak of the
American genera under the old term Heliconidae.

ix WARNING COLORATION AND MIMICRY 243

of the larvae or some other cause, possessed disagreeable
juices that caused them to be disliked by the usual enemies
of their kind, they were in all probability not very different
either in form or coloration from many other butterflies. They
would at that time be subject to repeated attacks by insect-
eaters, and, even if finally rejected, would often receive a
fatal injury. Hence arose the necessity for some distinguish-
ing mark, by which the devourers of butterflies in general
might learn that these particular butterflies were uneatable ;
and every variation leading to such distinction, whether by
form, colour, or mode of flight, was preserved and accumulated
by natural selection, till the ancestral Heliconoids became well
distinguished from eatable butterflies, and thenceforth com-
paratively free from persecution. Then they had a good
time of it. They acquired lazy habits, and flew about slowly.
They increased abundantly and spread all over the country,
their larvae feeding on many plants and acquiring different
habits ; while the butterflies themselves varied greatly, and
colour being useful rather than injurious to them, gradually
diverged into the many coloured and beautifully varied forms
we now behold.

But, during the early stages of this process, some of the
Pieridse, inhabiting the same district, happened to be sufficiently
like some of the Heliconidse to be occasionally mistaken for
them. These, of course, survived while their companions were
devoured. Those among their descendants that were still more
like Heliconidse again survived, and at length the imitation
would become tolerably perfect. Thereafter, as the protected
group diverged into distinct species of many different colours,
the imitative group would occasionally be able to follow it
with similar variations, — a process that is going on now, for
Mr. Bates informs us that in each fresh district he visited he
found closely allied representative species or varieties of
Heliconidse, and along with them species of Leptalis
(Pieridse), which had varied in the same way so as still to be
exact imitations. But this process of imitation would be
subject to check by the increasing acuteness of birds and other
animals which, whenever the eatable Leptalis became numerous,
would surely find them out, and would then probably attack
both these and their friends the Heliconidse in order to devour

244 DARWINISM chap.

the former and reject the latter. The Pieridae would, however,
usually be less numerous, because their larvae are often pro-
tectively coloured and therefore edible, while the larva? of the
Heliconidae are adorned with warning colours, spines, or
tubercles, and are uneatable. It seems probable that the
larvae and pupae' of the Heliconidae were the first to acquire
the protective distastefulness, both because in this stage they
are more defenceless and more liable to fatal injury, and also
because we now find many instances in which the larvae are
distasteful while the perfect insects are eatable, but I believe
none in which the reverse is the case. The larvae of the
Pieridae are now beginning to acquire offensive juices, but
have not yet obtained the corresponding conspicuous colours ;
while the perfect insects remain eatable, except perhaps in
some Eastern groups, the under sides of whose wings are
brilliantly coloured although this is the part which is exposed
when at rest.

It is clear that if a large majority of the larvae of Lepido-
ptera, as well as the perfect insects, acquired these distaste-
ful properties, so as seriously to diminish the food supply of
insectivorous and nestling birds, these latter would be forced
by necessity to acquire corresponding tastes, and to eat with
pleasure what some of them now eat only under pressure of
hunger ; and variation and natural selection would soon bring
about this change.

Many writers have denied the possibility of such wonderful
resemblances being produced by the accumulation of fortuitous
variations, but if the reader will call to mind the large amount
of variability that has been shown to exist in all organisms,
the exceptional power of rapid increase possessed by insects, and
the tremendous struggle for existence always going on, the
difficulty will vanish, especially when we remember that
nature has the same fundamental groundwork to act upon in
the two groups, general similarity of forms, wings of similar
texture and outline, and probably some original similarity of
colour and marking. Yet there is evidently considerable
difficulty in the process, or with these great resources at her
command nature would have produced more of these mimicking
forms than she has done. One reason of this deficiency prob-
ably is, that the imitators, being always fewer in number, have

ix WARNING COLORATION AND MIMICRY 245

not been able to keep pace with the variations of the much
more numerous imitated form ; another reason may be the
ever -increasing acuteness of the enemies, which have again
and again detected the imposture and exterminated the
feeble race before it has had time to become further modified.
The result of this growing acuteness of enemies has been,
that those mimics that now survive exhibit, as Mr. Bates well
remarks, "a palpably intentional likeness that is perfectly
staggering," and also " that those features of the portrait are
most attended to by nature which produce the most effective
deception when the insects are seen in nature." No one, in
fact, can understand the perfection of the imitation who has
not seen these species in their native wilds. So complete
is it in general effect that in almost every box of butterflies,
brought from tropical America by amateurs, are to be found
some species of the mimicking Pieridse, Erycinidsej or moths,
and the mimicked Heliconidse, placed together under the
impression that they are the same species. Yet more ex-
traordinary, it sometimes deceives the very insects themselves.
Mr. Trimen states that the male Danais chrysippus is some-
times deceived by the female Diadema bolina which mimics
that species. Dr. Fritz Midler, writing from Brazil to Professor
Meldola, says, " One of the most interesting of our mimick-
ing butterflies is Leptalis melite. The female alone of this
species imitates one of our common white Piericlse, which she
copies so well that even her own male is often deceived ; for
I have repeatedly seen the male pursuing the mimicked
species, till, after closely approaching and becoming aware of
his error, he suddenly returned."1 This is evidently not a
case of true mimicry, since the species imitated is not pro-
tected ; but it may be that the less abundant Leptalis is able
to mingle with the female Pieridse and thus obtain partial
immunity from attack. Mr. Kirby of the insect department
of the British Museum informs me that there are several
species of South American Pieridse which the female Leptalis
melite very nearly resembles. The case, however, is interest-
ing as showing that the butterflies are themselves deceived by
a resemblance which is not so great as that of some mimicking
species.

1 E. Meldola in Ann. and Mar;, of Nat. Hist, Feb. 1878, p. 158.

246 DARWINISM

Other Examples of Mimicry among Lepidoptera.

In tropical Asia, and eastward to the Pacific Islands, the
Danaidse take the place of the Heliconidae of America, in their
abundance, their conspicuousness, their slow flight, and their
being the subjects of mimicry. They exist under three
principal forms or genera. The genus Euplcea is the most
abundant both in species and individuals, and consists of fine
broad-winged butterflies of a glossy or metallic blue-black
colour, adorned with pure white, or rich blue, or dusky mark-
ings situated round the margins of the wings. Danais has
generally more lengthened wings, of a semitransparent greenish
or a rich brown colour, with radial or marginal pale spots ;
while the fine Hestias are of enormous size, of a papery or
semitransparent white colour, with dusky or black spots and
markings. Each of these groups is mimicked by various
species of the genus Papilio, usually with such accuracy that
it is impossible to distinguish them on the wing.1 Several
species of Diadema, a genus of butterflies allied to our
Vanessas, also mimic species of Danais, but in this case the
females only are affected, a subject which will be discussed in
another chapter.

Another protected group in the Eastern tropics is that of
the beautiful day-flying moths forming the family Agaristidse.
These are usually adorned with the most brilliant colours or
conspicuous markings, they fly slowly in forests among the
butterflies and other diurnal insects, and their great abundance
sufficiently indicates their possession of some distastefulness
which saves them from attack. Under these conditions we
may expect to find other moths which are not so protected
imitating them, and this is the case. One of the common and
wide-ranging species (Opthalmis lincea), found in the islands
from Amboyna to New Ireland, is mimicked in a wonderful
manner by one of the Liparidse (the family to which our
common " tussock " and " vapour er " moths belong). This is
a new species collected at Amboyna during the voyage of the
Challenger, and has been named Artaxa simulans. Both

i See Trans. Linn. Soc., vol. xxv. Wallace, on Variation of Malayan
Papilionida? ; and, Wallace's Contributions to Natural Selection, chaps, iii. and
iv., where full details are given.

IX WARNING COLORATION AND MIMICRY 247

insects are black, Avith the apex of the fore wings ochre coloured,
and the outer half of the hind wings bright orange. The
accompanying woodcuts (for the use of which I am indebted
to Mr. John Murray of the Challenger Office) well exhibit their
striking resemblance to each other.

Pig. 24. — Opthalmis lincea (Agaristicla?). Artaxa simulans (Liparidce).

In Africa exactly similar phenomena recur, species of Papilio
and of Diadema mimicking Danaidae or Acraeidae with the
most curious accuracy. Mr. Trimen, who studied this subject
in South Africa, has recorded eight species or varieties of
Diadema, and eight of Papilio, which each mimic some
species of Danais ; while eight species or varieties of Panopaea
(another genus of Nymphalidae), three of Melanitis (Eury-
telidse), and two of Papilio, resemble with equal accuracy
some species of Acraea.1 He has also independently observed
the main facts on which the explanation of the phenomenon
rests, — the unpleasant odour of the Danais and Acraea, extend-
ing to their larvae and pupae ; their great abundance, slow
flight, and disregard of concealment ; and he states that while
lizards, mantidse, and dragonflies all hunt butterflies, and the
rejected wings are to be found abundantly at some of their

1 See Trans. Linn. Soc, vol. xxvi.. with two coloured plates illustrating
cases of mimicry.

248 DARWINISM

feeding-places, those of the two genera Danais and Acrsea
were never among them.

The two groups of the great genus Papilio (the true swallow-
tailed butterflies) which have been already referred to as
having the special characteristics of uneatable insects, have also
their imitators in other groups : and thus, the belief in
their inedibility — derived mainly from their style of warning
coloration and their peculiar habits — is confirmed. In South
America, several species of the " iEneas " group of these
butterflies are mimicked by Pieridse and by day-flying moths
of the genera Castnia and Pericopis. In the East, Papilio
hector, P. diphilus, and P. liris, all belonging to the inedible
group, are mimicked by the females of other species of Papilio
belonging to very distinct groups ; while in Northern India
and China, many fine day -flying moths (Epicopeia) have ac-
quired the strange forms and peculiar colours of some of the
large inedible Papilios of the same regions.

In North America, the large and handsome Danais
archippus, with rich reddish-brown wings, is very common ;
and it is closely imitated by Limenitis misippus, a butterfly
allied to our " white admiral," but which has acquired a colour
quite distinct from that of the great bulk of its allies. In
the same country there is a still more interesting case. The
beautiful dark bronzy green butterfly, Papilio philenor, is
inedible both in larva and perfect insect, and it is mimicked
by the equally dark Limenitis Ursula. There is also in the
Southern and Western States a dark female form of the yellow
Papilio turnus, which in all probability obtains protection from
its general resemblance to P. philenor. Mr. W. H. Edwards
has found, by extensive experiment, that both the dark and
yellow females produce their own kinds, with very few excep-
tions ; and he thinks that the dark form has the advantage in
the more open regions and in the prairies, where insectivorous
birds abound. But in open country the dark form would
be quite as conspicuous as the yellow form, if not more so,
so that the resemblance to an inedible species would be there
more needed.1

The only probable case of mimicry in this country is that
of the moth, Diaphora mendica, whose female only is white,

1 Edwards's Butterflies of North America, second series, part vi.

IX WARNING COLORATION AND MIMICRY 249.

while the larva is of protective colours, and therefore almost
certainly edible. A much more abundant moth, of about the
same size and appearing about the same time, is Spilosoma
menthrasti, also white, but in this case both it and its larva
have been proved to be inedible. The white colour of the
female Diaphora, although it must be very conspicuous at
night, may, therefore, have been acquired in order to re-
semble the uneatable Spilosoma, and thus gain some pro-
tection.1

Mimicry among Protected ( Uneatable) Genera.

Before giving some account of the numerous other cases
of warning colours and of mimicry that occur in the animal
kingdom, it will be well to notice a curious phenomenon
which long puzzled entomologists, but which has at length
received a satisfactory explanation.

We have hitherto considered, that mimicry could only occur
when a comparatively scarce and much persecuted species
obtained protection by its close external resemblance to a
much more abundant uneatable species inhabiting its own dis-
trict ; and this rule undoubtedly prevails among the great
majority of mimicking species all over the world. But Mr.
Bates also found a number of pairs of species of different genera
of Heliconidse, which resembled each other quite as closely as
did the other mimicking species he has described ; and since
all these insects appear to be equally protected by their in-
edibility, and to be equally free from persecution, it was not
easy to see why this curious resemblance existed, or how it
had been brought about. That it is not due to close affinity
is shown by the fact that the resemblance occurs most fre-
quently between the two distinct sub-families into which (as
Mr. Bates first pointed out) the Heliconidse are naturally
divided on account of very important structural differences.
One of these sub-families (the true Heliconinse) consists of two
genera only, Heliconius and Eueides, the other (the Danaoid
Heliconinse) of no less than sixteen genera ; and, in the in-
stances of mimicry we are now discussing, one of the pairs or

1 Professor Meldola informs me that he has recorded another case of
mimicry among British moths, in which Acidalia snbsericata imitates Asthena
candidata. See Eni. Mo. Mag., vol. iv. p. 163.

250 DARWINISM

triplets that resemble each other is usually a species of the large
and handsome genus Heliconius, the others being species of
the genera Mechanitis, Melinsea, or Tithorea, though several
species of other Danaoid genera also imitate each other. The
following lists will give some idea of the number of these
curious imitative forms, and of their presence in every part of
the Neotropical area. The bracketed species are those that
resemble each other so closely that the difference is not per-
ceptible when they are on the wing.

In the Lower Amazon region are found —

( Heliconius sylvana.
\ Melinsea egina.

( Heliconius numata.
-J Melinsea mneme.
^ Tithorea harmonia.

I Methona psidii.
\ Thyridia ino.

Ceratina ninonia.
Melinsea mnasias.

In Central America are found —

{Heliconius zuleika.
Melinsea hezia.
Mechanitis sp.

Heliconius formosus,
Tithorea penthias.

n . i \ Heliconius telchina.
Guatemala M ,. . . , ,
{ Melinsea mntata.

In the Upper Amazon region —

Heliconius pardalinus.
Melinsea pardalis.

Heliconius aurora.
Melinsea lucifer.

In New Grenada —

( Heliconius ismenius,
( Melinsea messatis.
f Heliconius mcssene.
-J Melinsea mesenina.
[(?) Mechanitis sp.
j Heliconius hecalesia.
j Tithorea liecalesina.
j Heliconius hecuba.
( Tithorea bonplandi.

WARNING COLORATION AND MIMICRY

251

In Eastern Pern and Bolivia —

f Heliconius aristona.
-! Melintea cydippe.
(_ (?) Mechanitis mothone.

In Pernambuco —

Heliconius ethra.
Mechanitis nessea.

In Eio Janeiro —

Heliconius eucrate.
Mechanitis lysimnia.

In South Brazil —

\ Thyridia megisto.
| Itunailione.
( Acrsea thalia.
( Eueides pavana.

Besides these, a number of species of Ithomia and Napeo-
genes, and of Napeogenes and Mechanitis, resemble each other
with equal accuracy, so that they are liable to be mistaken

Fig. 25. — Wings of Ituna Ilione, J .

Wings of Thyridia megisto, & .

for each other when on the wing ; and no doubt many other
equally remarkable cases are yet unnoticed.

The figures above of the fore and hind wings of tAvo of these
mimicking species, from Dr. Fritz Midler's original paper in
Kosmos, will serve to show the considerable amount of

252 DARWINISM

difference, in the important character of the neuration of the
wings, between these butterflies, which really belong to very-
distinct and not at all closely allied genera. Other important
characters are — (1) The existence of a small basal cell in the
hind wings of Ituna which is wanting in Thyridia ; (2) the
division of the cell between the veins lb and 2 of the
hind wings in the former genus, while it is undivided in the
latter ; and (3) the existence in Thyridia of scent-producing
tufts of hair on the upper edge of the hind wing, while in
Ituna these are wanting ; but in place of them are extensible
processes at the end of the abdomen, also emitting a powerful
scent. These differences characterise two marked subdivisions
of the Danaoid Heliconinae, each containing several distinct
genera; and these subdivisions are further distinguished by very
different forms of larvae, that to which Ituna belongs having
from two to four long threadlike tentacles on the back, while
in that containing Thyridia these are always absent. The
former usually feed on Asclepiadese, the latter on Solanacea?
or Scrophulariacese.

The two species figured, though belonging to such distinct
and even remote genera, have acquired almost identical tints
and markings so as to be deceptively alike. The surface of
the wings is, in both, transparent yellowish, with black trans-
verse bands and white marginal spots, while both have similar
black- and white-marked bodies and long yellow antennae.
Dr. Miiller states that they both show a preference for the
same flowers growing on the edges of the forest paths.1

We will now proceed to give the explanation of these
curious similarities, which have remained a complete puzzle
for twenty years. Mr. Bates, when first describing them,
suggested that they might be due to some form of parallel
variation dependent on climatic influences ; and I myself
adduced other cases of coincident local modifications of
colour, which did not appear to be explicable by any form
of mimicry.2 But we neither of us hit upon the simple
explanation given by Dr. Fritz Miiller in 1879.

His theory is founded on the assumed, but probable,

1 From Professor Meldola's translation of Dr. F. Miiller's paper, in Proc.
Ent. Soc. Lond., 1879, p. xx.

2 Island Life, p. 255.

ix WARNING COLORATION AND MIMICRY 253

fact, that insect-eating birds only learn by experience to
distinguish the edible from the inedible butterflies, and in
doing so necessarily sacrifice a certain number of the latter.
The quantity of insectivorous birds in tropical America is
enormous ; and the number of young birds which every year
have to learn wisdom by experience, as regards the species of
butterflies to be caught or to be avoided, is so great that the
sacrifice of life of the inedible species must be considerable,
and, to a comparatively weak or scarce species, of vital im-
portance. The number thus sacrificed will be fixed by the
quantity of young birds, and by the number of experiences
requisite to cause them to avoid the inedible species for the
future, and not at all by the numbers of individuals of which
each species consists. Hence, if two species are so much
alike as to be mistaken for one another, the fixed number
annually sacrificed by inexperienced birds will be divided be-
tween them, and both will benefit. But if the two species are
very unequal in numbers, the benefit will be comparatively
slight for the more abundant species, but very great for the rare
one. To the latter it may make all the difference between
safety and destruction.

To give a rough numerical example. Let us suppose that
in a given limited district there are two species of Heliconicla?,
one consisting of only 1000, the other of 100,000 individuals,
and that the quota required annually in the same district for
the instruction of young insectivorous birds is 500. By the
larger species this loss will be hardly felt ; to the smaller it
will mean the most dreadful persecution resulting in a
loss of half the total population. But, let the two species
become superficially alike, so that the birds see no difference
between them. The quota of 500 will now be taken from a
combined population of 101,000 butterflies, and if propor-
tionate numbers of each suffer, then the weak species will
only lose five individuals instead of 500 as it did before.
Now we know that the different species of Heliconidae are
not equally abundant, some being quite rare ; so that the
benefit to be derived in these latter cases would be very im-
portant. A slight inferiority in rapidity of flight or in powers
of eluding attack might also be a cause of danger to an in-
edible species of scanty numbers, and in this case too the being

254 DARWINISM

merged in another much more abundant species, by similarity
of external appearance, would be an advantage.

The question of fact remains. Do young birds pursue and
capture these distasteful butterflies till they have learned by
bitter experience what species to avoid 1 On this point Dr.
Miiller has fortunately been able to obtain some direct evi-
dence, by capturing several Acraeas and Heliconidae which had
evidently been seized by birds but had afterwards escaped, as
they had pieces torn out of the wing, sometimes symmetri-
cally out of both wings, showing that the insect had been
seized when at rest and with the two pairs of wings in contact.
There is, however, a general impression that this knowledge is
hereditary, and does not need to be acquired by young birds ;
in support of which view Mr. Jenner Weir states that his birds
always disregarded inedible caterpillars. When, day by day,
he threw into his aviary various larvae, those which were
edible were eaten immediately, those which were inedible
were no more noticed than if a pebble had been thrown
before the birds.

The cases, however, are not strictly comparable. The
birds Avere not young birds of the first year; and, what
is more important, edible larvae have a comparatively simple
coloration, being always brown or green and smooth. Uneat-
able larvae, on the other hand, comprise all that are of conspicu-
ous colours and are hairy or spiny. But with butterflies there
is no such simplicity of contrast. The eatable butterflies com-
prise not only brown or white species, but hundreds of
Nymphalidae, Papilionidae, Lycaenidae, etc., which are gaily
coloured and of an immense variety of patterns. The colours
and patterns of the inedible kinds are also greatly Araried,
while they are often equally gay; and it is quite impossible
to suppose that any amount of instinct or inherited habit
(if such a thing exists) could enable young insectivorous
birds to distinguish all the species of one kind from all
those of the other. There is also some evidence to show
that animals do learn by experience what to eat and what
to avoid. Mr. Poulton was assured by Rev. Gr. J. Bursch
that very young chickens peck at insects which they after-
wards avoid. Lizards, too, often seized larvae which they were
unable to eat and ultimately rejected.

ix WARNING COLORATION AND MIMICRY 255

Although the Heliconidse present, on the whole, many
varieties of coloration and pattern, yet, in proportion to the
number of distinct species in each district, the types of
coloration are few and very well marked, and thus it becomes
easier for a bird or other animal to learn that all belonging to
such types are uneatable. This must be a decided advantage to
the family in question, because, not only do fewer individuals
of each species need to be sacrificed in order that their enemies
may learn the lesson of their inedibility, but they are more
easily recognised at a distance, and thus escape even pursuit.
There is thus a kind of mimicry between closely allied species
as well as between species of distinct genera, all tending to the
same beneficial end. This may be seen in the four or five
distinct species of the genus Heliconius which all have the same
peculiar type of coloration — a yellow band across the upper
wings and radiating red stripes on the lower, — and are all found
in the same forests of the Lower Amazon ; in the numerous
very similar species of Ithomia with transparent wings, found
in every locality of the same region ; and in the very numerous
species of Papilio of the " .ZEneas " group, all having a similar
style of marking, the resemblance being especially close in
the females. The very uniform type of colouring of the
blue -black Euplseas and of the fulvous Acrseas is of the
same character.1 In all these cases the similarity of the
allied species is so great, that, when they are on the wing
at some distance off, it is difficult to distinguish one species
from another. But this close external resemblance is not
always a sign of very near affinity ; for minute examination
detects differences in the form and scalloping of the wings, in
the markings on the body, and in those on the under surface
of the wings, which do not usually characterise the closest allies.
It is to be further noted, that the presence of groups of very
similar species of the same genus, in one locality, is not at all
a common phenomenon among unprotected groups. Usually
the species of a genus found in one locality are each well
marked and belong to somewhat distinct types, while the

1 This extension of the theory of mimicry was pointed out by Professor
Meklola in the paper already referred to ; and he has answered the objections
to Dr. F. Midler's theory with great force in the Annals and Mag. of Nat.
Hist., 1882, p. 417.

256 DARWINISM chap.

closely allied forms — those that require minute examination
to discriminate them as distinct species — are most generally
found in separate areas, and are what are termed representative
forms.

The extension we have now given to the theory of mimicry
is important, since it enables us to explain a much wider
range of colour phenomena than those which were first im-
puted to mimicry. It is in the richest butterfly region in the
world — the Amazon valley — that we find the most abundant
evidence of the three distinct sets of facts, all depending on
the same general principle. The form of mimicry first
elucidated by Mr. Bates is characterised by the presence in
each locality of certain butterflies, or other insects, themselves
edible and belonging to edible groups, which derived protec-
tion from having acquired a deceptive resemblance to some
of the inedible butterflies in the same localities, which latter
were believed to be wholly free from the attacks of in-
sectivorous birds. Then came the extension of the principle,
by Dr. F. Midler, to the case of species of distinct genera
of the inedible butterflies resembling each other quite as
closely as in the former cases, and like them always found
in the same localities. They derive mutual benefit from
becoming, in appearance, one species, from which a certain
toll is taken annually to teach the young insectivorous birds
that they are uneatable. Even when the two or more species
are approximately equal in numbers, they each derive a
considerable benefit from thus combining their forces ; but
when one of the species is scarce or verging on extinction, the
benefit becomes exceedingly great, being, in fact, exactly appor-
tioned to the need of the species.

The third extension of the same principle explains the
grouping of allied species of the same genera of inedible
butterflies into sets, each having a distinct type of coloration,
and each consisting of a number of species Avhich can hardly
be distinguished on the wing. This must be useful exactly
in the same way as in the last case, since it divides the
inevitable toll to insectivorous birds and other animals
among a number of species. It also explains the fact of the
great similarity of many species of inedible insects in the
same locality — a similarity which does not obtain to anything

ix WARNING COLORATION AND MIMICRY 257

like the same extent among the edible species. The explana-
tion of the various phenomena of resemblance and mimicry,
presented by the distasteful butterflies, may now be considered
tolerably complete.

Mimicry in other Orders of Insects.

A very brief sketch of these phenomena will be given,
chiefly to show that the same principle prevails throughout
nature, and that, wherever a rather extensive group is
protected, either by distastefulness or offensive weapons,
there are usually some species of edible and inoffensive
groups that gain protection by imitating them. It has been
already stated that the Telephoridse, Lampyridse, and other
families of soft -winged beetles, are distasteful; and as they
abound in all parts of the world, and especially in the tropics,
it is not surprising that insects of many other groups should
imitate them. This is especially the case with the longicorn
beetles, which are much persecuted by insectivorous birds ; and
everywhere in tropical regions some of these are to be found
so completely disguised as to be mistaken for species of the
protected groups. Numbers of these imitations have been
already recorded by Mr. Bates and myself, but I will here
refer to a few others.

In the recently published volumes on the Longicorn and
Malacoderm beetles of Central America x there are numbers of
beautifully coloured figures of the new species ; and on looking
over them we are struck by the curious resemblance of some
of the Longicorns to species of the Malacoderm group. In
some cases we discover perfect mimics, and on turning to the
descriptions we always find these pairs to come from the
same locality. Thus the Otheostethus melanurus, one of the
Prionidse, imitates the malacoderm, Lucidota discolor, in
form, peculiar coloration, and size, and both are found at
Chontales in Nicaragua, the species mimicked having, how-
ever, as is usual, a wider range. The curious and very rare
little longicorn, Tethlimmena aliena, quite unlike its nearest
allies in the same country, is an exact copy on a somewhat
smaller scale of a malacoderm, Lygistopterus amabilis, both

1 Godman and Salvin's Biologia Centrali- American a, Insecta, Coleojrtera,
vol. iii. part ii., and vol. v.

S

258 DARWINISM

found at Chontales. The pretty longicorn, Callia albieornis,
closely resembles two species of malacoderms (Silis chaly-
beipennis and Colyphus signaticollis), all being small beetles
with red head and thorax and bright blue elytra, and all
three have been found at Panama. Many other species of
Callia also resemble other malacoderms ; and the longicorn
genus Lycidola has been named from its resemblance to
various species of the Lycidse, one of the species here figured
(Lycidola belti) being a good mimic of Calopteron corrugatum
and of several other allied species, all being of about the same
size and found at Chontales. In these cases, and in most
others, the longicorn beetles have lost the general form and
aspect of their allies to take on the appearance of a distinct
tribe. Some other groups of beetles, as the Elateridse and
Eucnemidse, also deceptively mimic malacoderms.

Wasps and bees are often closely imitated by insects of
other orders. Many longicorn beetles in the tropics exactly
mimic wasps, bees, or ants. In Borneo a large black wasp,
whose wings have a broad white patch near the apex (Myg-
nimia aviculus), is closely imitated by a heteromerous beetle
(Coloborhombus fasciatipennis), which, contrary to the general
habit of beetles, keeps its wings expanded in order to show
the white patch on their apex, the wing-coverts being reduced
to small oval scales, as shown in the figure. This is a most
remarkable instance of mimicry, because the beetle has had to
acquire so many characters which are unknown among its allies
(except in another species from Java) — the expanded wings,
the white band on them, and the oval scale -like elytra.1
Another remarkable case has been noted by Mr. Neville
Goodman, in Egypt, where a common hornet (Vespa orientalis)
is exactly imitated in colour, size, shape, attitude when at
rest, and mode of flight, by a beetle of the genus Laphria.2

The tiger -beetles (Cicindelidse) are also the subjects of
mimicry by more harmless insects. In the Malay Islands I
found a heteromerous beetle which exactly resembled a
Therates, both being found running on the trunks of trees.
A longicorn (Collyrodes Lacordairei) mimics Collyris, another
genus of the same family ; while in the Philippine Islands

1 Trans. Ent. Soc, 1S85, p. 369.
2 Proc. Cambridge Phil. Soc, vol. iii. part ii., 1877.

ix WARNING COLORATION AND MIMICRY 259

there is a cricket (Condylodeira tricondyloides), which so
closely resembles a tiger -beetle of the genus Tricondyla

Fig. 26. — Mygnimia aviculus (Wasp). Coloborhombus fasciatipennis (Beetle).

that the experienced entomologist, Professor Westwood, at first
placed it in his cabinet among those beetles.

260

DARWINISM

One of the characters by which some beetles are protected
is excessive hardness of the elytra and integuments. Several
genera of weevils (Curculionidse) are thus saved from attack,
and these are often mimicked by species of softer and more

Fig. 27.

a. Doliops sp. (Longicorn) mimics Pachyrhynclms orbifae, (b) (a hard curculio).

c. Doliops curculionoides mimics (d) Pachyrhynclms sp.

e. Scepastus pachyrhynchoides (a grasshopper) mimics (/) Apocyrtus sp. (a hard

curculio).
g. Doliops sp. mimics (7i) Pachyrhynclms sp.
i. Phoraspis (grasshopper) mimics (k) a Coccinella.

All the above are from the Philippines. The exact correspondence of the colours
of the insects themselves renders the mimicry much more complete in nature than it
appears in the above figures.

eatable groups. In South America, the genus Heilipus is one
of these hard groups, and both Mr. Bates and M. Roelofs,
a Belgian entomologist, have noticed that species of other
genera exactly mimic them. So, in the Philippines, there

ix WARNING COLORATION AND MIMICRY 261

is a group of Curculionidse, forming the genus Pachyrhynchus,
in which all the species are adorned with the most brilliant
metallic colours, banded and spotted in a curious manner,
and are very smooth and hard. Other genera of Curculionidse
(Desmidophorus, Alcides), which are usually very differently
coloured, have species in the Philippines which mimic the
Pachyrhynchi ; and there are also several longicorn beetles
(Aprophata, Doliops, Acronia, and Agnia), which also mimic
them. Besides these, there are some longicorns and cetonias
which reproduce the same colours and markings ; and there
is even a cricket (Scepastus pachyrhynchoides), which has
taken on the form and peculiar coloration of these beetles
in order to escape from enemies, which then avoid them as
uneatable.1 The figures on the opposite page exhibit several
other examples of these mimicking insects.

Innumerable other cases of mimicry occur among tropical
insects ; but we must now pass on to consider a few of the
very remarkable, but much rarer instances, that are found
among the higher animals.

Mimicry among the Vertebrata.

Perhaps the most remarkable cases yet known are those of
certain harmless snakes which mimic poisonous species. The
genus Elaps, in tropical America, consists of poisonous snakes
which do not belong to the viper family (in which are included
the rattlesnakes and most of those which are poisonous), and
which do not possess the broad triangular head which charac-
terises the latter. They have a peculiar style of coloration,
consisting of alternate rings of red and black, or red, black,
and yellow, of different widths and grouped in various ways
in the different species ; and it is a style of coloration which
does not occur in any other group of snakes in the world.
But in the same regions are found three genera of harmless
snakes, belonging to other families, some few species of which
mimic the poisonous Elaps, often so exactly that it is with
difficulty one can be distinguished from the other. Thus
Elaps fulvius in Guatemala is imitated by the harmless Pro-
cerus equalis ; Elaps corallinus in Mexico is mimicked by the

1 Corrvpte- Rendu de la Societe Entomologique de Belgaue, series ii.; No. 59,
1878.

262 DARWINISM

harmless Homalocranium semicinctum ; and Elaps lemniscatus
in Brazil is copied by Oxyrhopus trigeminus ; while in other
parts of South America similar cases of mimicry occur, some-
times two harmless species imitating the same poisonous
snake.

A few other instances of mimicry in this group have been
recorded. There is in South Africa an egg -eating snake
(Dasypeltis scaber), which has neither fangs nor teeth, yet it is
very like the Berg adder (Clothos atropos), and when alarmed
renders itself still more like by flattening out its head and
darting forward with a hiss as if to strike a foe.1 Dr. A. B.
Meyer has also discovered that, while some species of the
genus Callophis (belonging to the same family as the American
Elaps) have large poison fangs, other species of the same genus
have none ; and that one of the latter (C. gracilis) resembles
a poisonous species (C. intestinalis) so closely, that only an
exact comparison will discover the difference of colour and
marking. A similar kind of resemblance is said to exist
between another harmless snake, Megaerophis flaviceps, and
the poisonous Callophis bivirgatus ; and in both these cases
the harmless snake is less abundant than the poisonous one,
as occurs in all examples of true mimicry.2

In the genus Elaps, above referred to, the very peculiar
style of colour and marking is evidently a " warning colour "
for the purpose of indicating to snake-eating birds and mam-
mals that these species are poisonous ; and this throAvs light on
the long -disputed question of the use of the rattle of the
rattlesnake. This reptile is really both sluggish and timid,
and is very easily captured by those who know its habits. If
gently tapped on the head with a stick, it will coil itself up
and lie still, only raising its tail and rattling. It may then
be easily caught. This shows that the rattle is a warning to
its enemies that it is dangerous to proceed to extremities ;
and the creature has probably acquired this structure and
habit because it frequents open or rocky districts where
protective colour is needful to save it from being pounced
upon by buzzards or other snake- eaters. Quite parallel
in function is the expanded hood of the Indian cobra, a

1 Nature, vol. xxxiv. p. 547.
2 Proceedings of the Zool. Soc. of London, 1870, p. 369.

ix WARNING COLORATION AND MIMICRY 263

poisonous snake which belongs also to the Elapidae. This
is, no doubt, a warning to its foes, not an attempt to
terrify its prey ; and the hood has been acquired, as in the
case of the rattlesnake, because, protective coloration being
on the whole useful, some mark was required to distinguish
it from other protectively coloured, but harmless, snakes.
Both these species feed on active creatures capable of escaping
if their enemy were visible at a moderate distance.

Mimicry among Birds.

The varied forms and habits of birds do not favour the
production among them of the phenomena of warning colours
or of mimicry ; and the extreme development of their instincts
and reasoning powers, as well as their activity and their
power of flight, usually afford them other means of evading
their enemies. Yet there are a few imperfect, and one or
two very perfect cases of true mimicry to be found among
them. The less perfect examples are those presented by
several species of cuckoos, an exceedingly weak and de-
fenceless group of birds. Our own cuckoo is, in colour and
markings, very like a sparrow-hawk. In the East, several
of the small black cuckoos closely resemble the aggressive
drongo- shrikes of the same country, and the small metallic
cuckoos are like glossy starlings ; while a large ground-
cuckoo of Borneo (Carpococcyx radiatus) resembles one of
the fine pheasants (Euplocamus) of the same country, both in
form and in its rich metallic colours.

More perfect cases of mimicry occur between some of the
dull-coloured orioles in the Malay Archipelago and a genus of
large honey-suckers — the Tropidorhynchi or "Friar- birds."
These latter are powerful and noisy birds which go in small
flocks. They have long, curved, and sharp beaks, and power-
ful grasping claws ; and they are quite able to defend them-
selves, often driving away crows and hawks which venture to
approach them too nearly. The orioles, on the other hand,
are weak and timid birds, and trust chiefly to concealment
and to their retiring habits to escape persecution. In each
of the great islands of the Austro-Malayan region there is a
distinct species of Tropidorhynchus, and there is always along
with it an oriole that exactly mimics it. All the Tropidorhynchi

264 DARWINISM

have a patch of bare black skin round the eyes, and a ruff
of curious pale recurved feathers on the nape, whence their
name of Friar-birds, the ruff being supposed to resemble the
cowl of a friar. These peculiarities are imitated in the orioles
by patches of feathers of corresponding colours ; while the dif-
ferent tints of the two species in each island are exactly the
same. Thus in Bouru both are earthy brown ; in Ceram they are
both washed with yellow ochre ; in Timor the under surface
is pale and the throat nearly white, and Mr. H. 0. Forbes has
recently discovered another pair in the island of Timor Laut.
The close resemblance of these several pairs of birds, of widely
different families, is quite comparable with that of many of the
insects already described. It is so close that the preserved
specimens have even deceived naturalists ; for, in the great
French work, Voyage de V Astrolabe, the oriole of Bouru is
actually described and figured as a honey-sucker; and Mr.
Forbes tells us that, when his birds were submitted to Dr.
Sclater for description, the oriole and the honey-sucker were,
previous to close examination, considered to be the same
species.

Objections to the Theory of Mimicry.

To set forth adequately the varied and surprising facts of
mimicry would need a large and copiously illustrated volume;
and no more interesting subject could be taken up by a
naturalist who has access to our great collections and can de-
vote the necessary time to search out the many examples of
mimicry that lie hidden in our museums. The brief sketch of
the subject that has been here given will, however, serve to
indicate its nature, and to show the weakness of the objections
that were at first made to it. It was urged that the action
of " like conditions," with " accidental resemblances " and
" reversion to ancestral types," would account for the facts. If,
however, we consider the actual phenomena as here set forth,
and the very constant conditions under which they occur, we
shall see how utterly inadequate are these causes, either
singly or combined. These constant conditions are —

1. That the imitative species occur in the same area and

occupy the very same station as the imitated.

2. That the imitators are always the more defenceless.

ix WARNING COLORATION AND MIMICRY 265

3. That the imitators are always less numerous in in-

dividuals.

4. That the imitators differ, from the bulk of their allies.

5. That the imitation, however minute, is external and

visible only, never extending to internal characters or
to such as do not affect the external appearance.

These five characteristic features of mimicry show us that
it is really an exceptional form of protective resemblance.
Different species in the same group of organisms may obtain
protection in different ways : some by a general resemblance
to their environment ; some by more exactly imitating the
objects that surround them — bark, or leaf, or flower ; while
others again gain an equal protection by resembling some
species which, from whatever cause, is almost as free from
attack as if it were a leaf or a flower. This immunity may
depend on its being uneatable, or dangerous, or merely strong ;
and it is the resemblance to such creatures for the purpose
of sharing in their safety that constitutes mimicry.

Concluding Remarks on Warning Colours and Mimicry.

Colours which have been acquired for the purpose of serv-
ing as a warning of inedibility, or of the possession of
dangerous offensive weapons, are probably more numerous than
have been hitherto supposed ; and, if so, we shall be able to
explain a considerable amount of colour in nature for which
no use has hitherto been conjectured. The brilliant and
varied colours of sea-anemones and of many coral animals
will probably come under this head, since we know that
many of them possess the power of ejecting stinging threads
from various parts of their bodies which render them quite
uneatable to most animals. Mr. Gosse describes how, on
putting an Anthea into a tank containing a half -grown
bullhead (Cottus bubalis) which had not been fed for some
time, the fish opened his mouth and sucked in the morsel,
but instantly shot it out again. He then seized it a second
time, and after rolling it about in his mouth for a moment
shot it out again, and then darted away to hide himself
in a hole. Some tropical fishes, however, of the genera
Tetrodon, Pseudoscarus, Astracion, and a few others, seem

266 DARWINISM

to have acquired the power of feeding on corals and medusae ;
and the beautiful bands and spots and bright colours with
which they are frequently adorned, may be either protective
when feeding in the submarine coral groves, or may, in some
cases, be warning colours to show that they themselves are
poisonous and unea'table.

A remarkable illustration of the wide extension of warning
colours, and their very definite purpose in nature, is afforded
by what may now be termed " Mr. Belt's frog." Frogs in all
parts of the world are, usually, protectively coloured with
greens or browns ; and the little tree-frogs are either green
like the leaves they rest upon, or curiously mottled to imitate
bark or dead leaves. But there are a certain number of very
gaily coloured frogs, and these do not conceal themselves as
frogs usually do. Such was the small toad found by Darwin
at Bahia Blanca, which was intense black and bright vermilion,
and crawled about in the sunshine over dry sand-hills and
arid plains. And in Nicaragua, Mr. Belt found a little frog
gorgeously dressed in a livery of red and blue, which did
not attempt concealment and was very abundant, a combina-
tion of characters which convinced him that it was uneatable.
He, therefore, took a few specimens home with him and
gave them to his fowls and ducks, but none would touch
them. At last, by throwing down pieces of meat, for
which there was a great competition among the poultry,
he managed to entice a young duck into snatching up
one of the little frogs. Instead of swallowing it, however,
the duck instantly threw it out of its mouth, and went
about jerking its head as if trying to get rid of some un-
pleasant taste.1

The power of predicting what will happen in a given case
is always considered to be a crucial test of a true theory,
and if so, the theory of warning colours, and with it that of
mimicry, must be held to be well established. Among the
creatures which probably have warning colours as a sign of
inedibility are, the brilliantly coloured nudibranchiate molluscs,
those curious annelids the Nereis and the Aphrodite or sea-
mouse, and many other marine animals. The brilliant colours
of the scallops (Pecten)and some other bivalve shells are perhaps
1 The Naturalist in Nicaragua, p. 321.

ix WARNING COLORATION AND MIMICRY 267

an indication of their hardness and consequent inedibility, as
in the case of the hard beetles ; and it is not improbable that
some of the phosphorescent fishes and other marine organisms
may, like the glow-worm, hold out their lamp as a warning to
enemies.1 In Queensland there is an exceedingly poisonous
spider, whose bite will kill a dog, and cause severe illness with
excruciating pain in man. It is black, with a bright vermilion
patch on the middle of the body ; and it is so well recognised
by this conspicuous coloration that even the spider-hunting
wasps avoid it.2

Locusts and grasshoppers are generally of green protective
tints, but there are many tropical species most gaudily
decorated with red, blue, and black colours. On the same
general grounds as those by which Mr. Belt predicted the in-
edibility of his conspicuous frog, we might safely predict the
same for^ these insects ; but we have fortunately a proof that
they are so protected, since Mr. Charles Home states that
one of the bright coloured Indian locusts was invariably
rejected when offered to birds and lizards.3

The examples now given lead us to the conclusion that
colours acquired for the purpose of serving as a danger-signal
to enemies are very widespread in nature, and, with the
corresponding colours of the species which mimic them,
furnish us with a rational explanation of a considerable
portion of the coloration of animals which is outside the
limits of those colours that have been acquired for either
protection or recognition. There remains, however, another
set of colours, chiefly among the higher animals, which, being
connected with some of the most interesting and most
disputed questions in natural history, must be discussed in a
separate chapter.

1 Mr. Belt first suggested this use of the light of the Lampyridae (fireflies
and glow-worms) — Naturalist in Nicaragua, p. 320. Mr. Verrill and
Professor Meldola made the same suggestion in the case of medusae and other
phosphorescent marine organisms (Nature, vol. xxx. pp. 281, 289).

2 W. E. Armit, in Nature, vol. xviii. p. 642.

3 Proc. Ent. Soe., 1869, p. xiii.

CHAPTER X

COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX

Sex colours in the mollusca and Crustacea — In insects — In butterflies and
moths — Probable causes of these colours — Sexual selection as a
supposed cause — Sexual coloration of birds — Cause of dull colours of
female birds — Relation of sex colour to nesting habits — Sexual colours
of other vertebrates — Sexual selection by the struggles of males —
Sexual characters due to natural selection — Decorative plumage of
males and its effect on the females — Display of decorative plumage by
the males — A theory of animal coloration — The origin of accessory
plumes — Development of accessory plumes and their display — The
effect of female preference will be neutralised by natural selection —
General laws of animal coloration — Concluding remarks.

In the preceding chapters we have dealt chiefly with the
coloration of animals as distinctive of the several species ;
and we have seen that, in an enormous number of cases, the
colours can be shown to have a definite purpose, and to be
useful either as a means of protection or concealment, of
warning to enemies, or of recognition by their own kind. We
have now to consider a subordinate but very widespread
phenomenon — the differences of colour or of ornamental
appendages in the two sexes. These differences are found to
have special relations with the three classes of coloration
above referred to, in many cases confirming the explanation
already given of their purport and use, and furnishing us with
important aid in formulating a general theory of animal
coloration.

In comparing the colours of the two sexes we find a perfect
gradation, from absolute identity of colour up to such extreme
difference that it is difficult to believe that the two forms
can belong to the same species ; and this diversity in the

en. x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 269

colours of the sexes does not bear any constant relation
to affinity or systematic position. In both insects and birds
we find examples of complete identity and extreme diversity
of the sexes ; and these differences occur sometimes in the
same tribe or family, and sometimes even in the same
genus.

It is only among the higher and more active animals that
sexual differences of colour acquire any prominence. In the
mollusca the two sexes, when separated, are always alike in
colour, and only very rarely present slight differences in the
form of the shell. In the extensive group of Crustacea the
two sexes as a rule are identical in colour, though there are
often differences in the form of the prehensile organs ; but in
a very few cases there are differences of colour also. Thus, in
a Brazilian species of shore-crab (Gelasimus) the female is
grayish-brown, while in the male the posterior part of the
cephalo-thorax is pure white, with the anterior part of a rich
green. This colour is only acquired by the males when they
become mature, and is liable to rapid change in a few
minutes to dusky tints.1 In some of the fresh-water fleas
(Daphnoidse) the males are ornamented with red and blue
spots, while in others similar colours occur in both sexes. In
spiders also, though as a rule the two sexes are alike in colour,
there are a few exceptions, the males being ornamented with
brilliant colours on the abdomen, while the female is dull
coloured.

Sexual Coloration in Insects.

It is only when we come to the winged insects that we find
any large amount of peculiarity in sexual coloration, and
even here it is only developed in certain orders. Flies (Dip-
tera), field-bugs (Hemiptera), cicadas (Homoptera), and the
grasshoppers, locusts, and crickets (Orthoptera) present very
few and unimportant sexual differences of colour ; but the last
two groups have special musical organs very fully developed
in the males of some of the species, and these no doubt enable
the sexes to discover and recognise each other. In some cases,
however, when the female is protectively coloured, as in the
well-known leaf -insects already referred to (p. 207), the male

1 Darwin's Descent of Man, p. 271.

270 DARWINISM

is smaller and much, less protectively formed and coloured.
In the bees and wasps (Hymenoptera) it is also the rule that
the sexes are alike in colour, though there are several cases
among solitary bees where they differ; the female being
black, and the male brown in Anthophora retusa, while in
Andrsena fulva the female is more brightly coloured than the
male. Of the great order of beetles (Coleoptera) the same
thing may be said. Though often so rich and varied in their
colours the sexes are usually alike, and Mr. Darwin was only
able to find about a dozen cases in which there was any con-
spicuous difference between them.1 They exhibit, however,
numerous sexual characters, in the length of the antennae, and
in horns, legs, or jaws remarkably enlarged or curiously modi-
fied in the male sex.

It is in the family of dragonflies (order Neuroptera) that
we first meet with numerous cases of distinctive sexual
coloration. In some of the Agrionidse the males have the
bodies rich blue and the wings black, while the females have
the bodies green and the wings transparent. In the North
American genus Hetserina the males alone have a carmine
spot at the base of each wing ; but in some other genera the
sexes hardly differ at all.

The great order of Lepidoptera, including the butterflies and
moths, affords us the most numerous and striking examples of
diversity of sexual colouring. Among the moths the differ-
ence is usually but slight, being manifested in a greater inten-
sity of the colour of the smaller winged male ; but in a few
cases there is a decided difference, as in the ghost -moth
(Hepialus humuli), in which the male is pure white, while the
female is yellow with darker markings. This may be a
recognition colour, enabling the female more readily to discover
her mate ; and this view receives some support from the fact
that in the Shetland Islands the male is almost as yellow as
the female, since it has been suggested that at midsummer,
when this moth appears, there is in that high latitude sufficient
twilight all night to render any special coloration unneces-
sary.2

Butterflies present us with a wonderful amount of sexual

1 Darwin's Descent of Man, p. 294, and footnote.
2 Nature, 1871, p. 489.

x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 27?.

difference of colour, in many cases so remarkable that the two
sexes of the same species remained for many years under
different names and were thought to be quite distinct species.
We find, however, every gradation from perfect identity to
complete diversity, and in some cases we are able to see a
reason for this difference. Beginning with the most extra-
ordinary cases of diversity — as in Diadema misippus, where the
male is black, ornamented Avith a large white spot on each
wing margined with rich changeable blue, while the female is
orange-brown with black spots and stripes — we find the
explanation in the fact that the female mimics an uneatable
Danais, and thus gains protection while laying its eggs on low
plants in company with that insect. In the allied species,
Diadema bolina, the females are also very different from the
males, but are of dusky brown tints, evidently protective and
very variable, some specimens having a general resemblance
to the uneatable Euplseas ; so that we see here some of the
earlier stages of both forms of protection. The remarkable
differences in some South American Pieridse are similarly
explained. The males of Pieris pyrrha, P. lorena, and
several others, are white with a few black bands and marginal
spots like so many of their allies, while the females are
gaily coloured with yellow and brown, and exactly resemble
some species of the uneatable Heliconidse of the same
district. Similarly, in the Malay Archipelago, the female
of Diadema anomala is glossy metallic blue, while the
male is brown; the reason for this reversal of the usual
rule being, that the female exactly mimics the brilliant
colouring of the common and uneatable Euplsea midamus,
and thus secures protection. In the fine Adolias dirtea, the
male is black with a few specks of ochre-yellow and a broad
marginal band of rich metallic greenish-blue, while the female
is brownish-black entirely covered with rows of ochre-yellow
spots. This latter coloration does not appear to be protective
when the insect is seen in the cabinet, but it really is so.
I have observed the female of this butterfly in Sumatra, where
it settles on the ground in the forest, and its yellow spots
so harmonise with the flickering gleams of sunlight on the
dead leaves that it can only be detected with the greatest
difficulty.

272 DARWINISM

A hundred other cases might be quoted in which the
female is either more obscurely coloured than the male, or
gains protection by imitating some inedible species ; and any
one who has watched these female insects flying slowly
along in search of the plants on which to deposit their
eggs, will understand how important it must be to them
not to attract the attention of insect- eating birds by too
conspicuous colours. The number of birds which capture
insects on the wing is much greater in tropical regions
than in Europe ; and this is perhaps the reason why many
of our showy species are alike, or almost alike, in both
sexes, while they are protectively coloured on the under side
which is exposed to view when they are at rest. Such are
our peacock, tortoise-shell, and red admiral butterflies ; while
in the tropics we more commonly find that the females are
less conspicuous on the upper surface even when protectively
coloured beneath.

We may here remark, that the cases already quoted prove
clearly that either male or female may be modified in colour
apart from the opposite sex. In Pieris pyrrha and its allies
the male retains the usual type of coloration of the whole
genus, while the female has acquired a distinct and peculiar
style of colouring. In Adolias dirtea, on the other hand,
the female appears to retain something like the primitive
colour and markings of the two sexes, modified perhaps for
more perfect protection ; while the male has acquired more and
more intense and brilliant colours, only showing his original
markings by the few small yellow spots that remain near the
base of the wings. In the more gaily coloured Pieridse, of
Avhich our orange-tip butterfly may be taken as a type, Ave see in
the female the plain ancestral colours of the group, while the
male has acquired the brilliant orange tip to its wings, prob-
ably as a recognition mark.

In those species in which the under surface is protectively
coloured, we often find the upper surface alike in both sexes,
the tint of colour being usually more intense in the male. But
in some cases this leads to the female being more conspicuous,
as in some of the Lycsenidre, where the female is bright blue
and the male of a blue so much deeper and soberer in tint as
to appear the less brilliantly coloured of the two.

x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 273

Probable Causes of these Colours.

In the production of these varied results there have prob-
ably been several causes at work. There seems to be
a constant tendency in the male of most animals — but
especially of birds and insects — to develop more and more
intensity of colour, often culminating in brilliant metallic blues
or greens or the most splendid iridescent hues ; while, at
the same time, natural selection is constantly at work, pre-
venting the female from acquiring these same tints, or
modifying her colours in various directions to secure pro-
tection by assimilating her to her surroundings, or by pro-
ducing mimicry of some protected form. At the same
time, the need for recognition must be satisfied ; and this
seems to have led to diversities of colour in allied species,
sometimes the female, sometimes the male undergoing
the greatest change according as one or other could be
modified with the greatest ease, and so as to interfere least
Avith the welfare of the race. Hence it is that sometimes
the males of allied species vary most, as in the different
species of Epicalia; sometimes the females, as in the magnifi-
cent green species of Ornithoptera and the " iEneas " group
of Papilio.

The importance of the two principles — the need of pro-
tection and recognition — in modifying the comparative colora-
tion of the sexes among butterflies, is beautifully illustrated
in the case of the groups which are protected by their dis-
tastefulness, and whose females do not, therefore, need the
protection afforded by sober colours.

In the great families, Heliconidse and Acrseidse, we find
that the two sexes are almost always alike ; and, in the very
few exceptions, that the female, though differently, is not less
gaily or less conspicuously coloured. In the Danaidse the same
general rule prevails, but the cases in which the male exhibits
greater intensity of colour than the female are perhaps more
numerous than in the other two families. There is, however, a
curious difference in this respect between the Oriental and
the American groups of distasteful Papilios with warning
colours, both of which are the subjects of mimicry. In the
Eastern groups — of which P. hector and P. coon may be taken

T

274 DARWINISM

as types — the two sexes are nearly alike, the male being
sometimes more intensely coloured and with fewer pale
markings ; but in the American groups — represented by P.
seneas, P. sesostris, and allies — there is a wonderful diversity,
the males having a rich green or bluish patch on the fore wings,
while the females have a band or spots of pure white, not
always corresponding in position to the green spot of the
males. There are, however, transitional forms, by which a
complete series can be traced, from close similarity to great
diversity of colouring between the sexes ; and this may .perhaps
be only an extreme example of the intenser colour and more
concentrated markings which are a very prevalent character-
istic of male butterflies.

There are, in fact, many indications of a regular succession
of tints in which colour development has occurred in the
various groups of butterflies, from an original grayish or
brownish neutral tint. Thus in the " iEneas " group of
Papilios we have the patch on the upper wings yellowish in
P. triopas, olivaceous in P. bolivar, bronzy-gray with a white
spot in P. erlaces, more greenish and buff in P. iphidamas,
gradually changing to the fine blue of P. brissonius, and the
magnificent green of P. sesostris. In like manner, the intense
crimson spots of the lower wings can be traced step by step
from a yellow or buff tint, Avhich is one of the most wide-
spread colours in the whole order. The greater purity and
intensity of colour seem to be usually associated with more
pointed wings, indicating greater vigour and more rapid flight.

Sexual Selection as a supposed Cause of Colour Development

Mr. Darwin, as is well known, imputed most of the
brilliant colours and varied patterns of butterflies' wings to
sexual selection — that is, to a constant preference, by female
butterflies, for the more brilliant males ; the colours thus
produced being sometimes transmitted to the males alone,
sometimes to both sexes. This view has always seemed to
me to be unsupported by evidence, while it is also quite
inadequate to account for the facts. The only direct evidence,
as set forth with his usual fairness by Mr. Darwin himself, is
opposed to his views. Several entomologists assured him
that, in moths, the females evince not the least choice of their

x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 275

partners ; and Dr. Wallace of Colchester, who has largely bred
the fine Bombyx cynthia, confirmed this statement. Among
butterflies, several males often pursue one female, and Mr.
Darwin says, that, unless the female exerts a choice the
pairing must be left to chance. But, surely, it may be the
most vigorous or most persevering male that is chosen, not
necessarily one more brightly or differently coloured, and
this will be true " natural selection." Butterflies have been
noticed to prefer some coloured flowers to others ; but that
does not prove, or even render probable, any preference for
the colour itself, but only for flowers of certain colours, on
account of the more agreeable or more abundant nectar
obtained from them. Dr. Schulte called Mr. Darwin's atten-
tion to the fact, that in the Diadema bolina the brilliant blue
colour surrounding the white spots is only visible when we
look towards the insect's head, and this is true of many of
the iridescent colours of butterflies, and probably depends
upon the direction of the striae on the scales. It is suggested,
however, that this display of colour will be seen by the
female as the male is approaching her, and that it has been
developed by sexual selection.1 But in the majority of cases
the males follow the female, hovering over her in a position
which would render it almost impossible for her to see the
particular colours or patterns on his upper surface ; to do so
the female should mount higher than the male, and fly
towards him — being the seeker instead of the sought, and this
is quite opposed to the actual facts. I cannot, therefore,
think that this suggestion adds anything whatever to the
evidence for sexual selection of colour by female butterflies.
This question will, however, be again touched upon after we
have considered the phenomena of sexual colour among the
vertebrata.

Sexual Coloration of Birds.

The general rule among vertebrates, as regards colour, is,
for the two sexes to be alike. This prevails, with only a few
exceptions, in fishes, reptiles, and mammalia; but in birds
diversity of sexual colouring is exceedingly frequent, and is,
not improbably, present in a greater or less degree in more

1 Darwin in Nature, 1880, p. 237.

276 DARWINISM chap.

than half of the known species. It is this class, therefore, that
will afford us the best materials for a discussion of the problem,
and that may perhaps lead us to a satisfactory explanation of
the causes to which sexual colour is due.

The most fundamental characteristic of birds, from our
present point of View, is a greater intensity of colour in the
male. This is the case in hawks and falcons ; in many
thrushes, warblers, and finches ; in pigeons, partridges, rails,
plovers, and many others. When the plumage is highly
protective or of dull uniform tints, as in many of the
thrushes and warblers, the sexes are almost or quite identical
in colour ; but when any rich markings or bright tints are
acquired, they are almost always wanting or much fainter in
the female, as we see in the black -cap among warblers, and
the chaffinch among finches.

It is in tropical regions, where from a variety of causes
colour has been developed to its fullest extent, that we find
the most remarkable examples of sexual divergence of colour.
The most gorgeously coloured birds known are the birds
of paradise, the chatterers, the tanagers, the humming-birds,
and the pheasant -tribe, including the peacocks. In all these
the females are much less brilliant, and, in the great majority
of cases, exceptionally plain and dull coloured birds. Not
only are the remarkable plumes, crests, and gorgets of the
birds of paradise entirely wanting in the females, but these
latter are usually without any bright colour at all, and rank no
higher than our thrushes in ornamental plumage. Of the
humming-birds the same may be said, except that the females
are often green, and sometimes slightly metallic, but from
their small size and uniform tints are never conspicuous.
The glorious blues and purples, the pure whites and intense
crimsons of the male chatterers are represented in the females
by olive- greens or dull browns, as are the infinitely varied
tints of the male tanagers. And in pheasants, the splendour
of plumage which characterises the males is entirely absent
in the females, which, though often ornamental, have always
comparatively sober and protective tints. The same thing
occurs with many other groups. In the Eastern tropics
are many brilliant birds belonging to the families of the
warblers, flycatchers, shrikes, etc., but the female is always

x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 277

much less brilliant than the male and often quite dull
coloured.

Cause of Dull Colours of Female Birds.

The reason of this phenomenon is not difficult to find, if
we consider the essential conditions of a bird's existence, and
the most important function it has to fulfil. In order
that the species may be continued, young birds must be pro-
duced, and the female birds have to sit assiduously on their
eggs. While doing this they are exposed to observation and
attack by the numerous clevourers of eggs and birds, and it is
of vital importance that they should be protectively coloured
in all those parts of the body which are exposed during in-
cubation. To secure this end all the bright colours and
showy ornaments which decorate the male have not been
acquired by the female, who often remains clothed in the
sober hues which were probably once common to the whole
order to which she belongs. The different amounts of colour
acquired by the females have no doubt depended on
peculiarities of habits and of environment, and on the
powers of defence or of concealment possessed by the species.
Mr. Darwin has taught us that natural selection cannot
produce absolute, but only relative perfection; and as a
protective colour is only one out of many means by which
the female birds are able to provide for the safety of their
young, those which are best endowed in other respects will
have been allowed to acquire more colour than those with
whom the struggle for existence is more severe.

JRelation of Sex Colour to Nesting Habits.

This principle is strikingly illustrated by the existence of
considerable numbers of birds in which both sexes are
similarly and brilliantly coloured, — in some cases as brilliantly
as the males of many of the groups above referred to. Such
are the extensive families of the kingfishers, the woodpeckers,
the toucans, the parrots, the turacos, the hangnests, the
starlings, and many other smaller groups, all the species of
which are conspicuously or brilliantly coloured, while in all
of them the females are either coloured exactly like the males,
or, when differently coloured, are equally conspicuous. When

278 DARWINISM

searching for some cause for this singular apparent exception
to the rule of female protective colouring, I came upon a fact
which beautifully explains it ; for in all these cases, without
exception, the species either nests in holes in the ground or in
trees, or builds a domed or covered nest, so as completely to
conceal the sitting - bird. We have here a case exactly
parallel to that of the butterflies protected by distasteful-
ness, whose females are either exactly like the males, or, if
different, are equally conspicuous. We can hardly believe
that so exact a parallel should exist between such remote
classes of animals, except under the influence of a general
law ; and, in the need of |Drotection by all defenceless animals,
and especially by most female insects and birds, we have such
a law, which has been proved to have influenced the colours
of a considerable proportion of the animal kingdom.1

The general relation which exists between the mode of
nesting and the coloration of the sexes in those groups of
birds which need protection from enemies, may be thus
expressed : When both sexes are brilliant or conspicuous,
the nest is such as to conceal the sitting-bird ; but when the
male is brightly coloured and the female sits exposed on the
nest, she is always less brilliant and generally of quite sober
and protective hues.

It must be understood that the mode of nesting has in-
fluenced the colour, not that the colour has determined the
mode of nesting ; and this, I believe, has been generally, though
not perhaps universally, the case. For we know that colour
varies more rapidly, and can be more easily modified and
fixed by selection, than any other character ; whereas habits,
especially when connected with structure, and when they
pervade a whole group, are much more persistent and more
difficult to change, as shown by the habit of the dog turning
round two or three times before lying clown, believed to be
that of the wild ancestral form which thus smoothed down
the herbage so as to form a comfortable bed. We see, too,
that the general mode of nesting is characteristic of whole
families differing widely in size, form, and colours. Thus, all
the kingfishers and their allies in every part of the world nest

1 See the author's Contributions to Natural Selection, chap. vii. in which
these facts were first brought forward.

x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 279

in holes, usually in "banks, but sometimes in trees. The
motmots and the puff-birds (Bucconidse) build in similar
places ; while the toucans, barbets, trogons, woodpeckers, and
parrots all make their nests in hollow trees. This habit,
pervading all the members of extensive families, must there-
fore be extremely ancient, more especially as it evidently
depends in some degree on the structure of the birds, the
bills, and especially the feet, of all these groups being unfitted
for the construction of Avoven arboreal nests.1 But in all
these families the colour varies greatly from species to species,
being constant only in the one character of the similarity of
the sexes, or, at all events, in their being equally conspicuous
even though differently coloured.

When I first put forward this view of the connection
between the mode of nesting and the coloration of female
birds, I expressed the law in somewhat different terms, which
gave rise to some misunderstanding, and led to numerous
criticisms and objections. Several cases were brought forward
in which the females were far less brilliant than the males,
although the nest was covered. This is the case with the
MaluriclaB, or superb warblers of Australia, in which the males
are very brilliant during the pairing season and the females
quite plain, yet they build domed nests. Here, there can be
little doubt, the covered nest is a protection from rain or from
some special enemies to the eggs ; while the birds themselves
are protectively coloured in both sexes, except for a short
time during the breeding season when the male acquires
brilliant colours ; and this is probably connected with the fact
of their inhabiting the open plains and thin scrub of Australia,
where protective colours are as generally advantageous as
they are in our north-temperate zones.

As I have now stated the law, I do not think there are
any exceptions to it, while there are an overwhelming number
of cases which give it a strong support. It has been objected
that the domed nests of many birds are as conspicuous as the
birds themselves would be, and would, therefore, be of no use
as a protection to the birds and young. But, as a matter of
fact, they do protect from attack, for hawks or crows do not
pluck such nests to pieces, as in doing so they would be
1 On this point see the author's Contributions to Natural Selection, chap. v. i.

280 DARWINISM

exposed to the attack of the whole colony ; whereas a hawk or
falcon could carry off a sitting-bird or the young at a swoop,
and entirely avoid attack. Moreover, each kind of covered
nest is doubtless directed against the attacks of the most
dangerous enemies of the species, the purse-like nests, often a
yard long, suspended from the extremity of thin twigs, being
useful against the attacks of snakes, Avhich, if they attempted
to enter them, would be easily made to lose their hold and
fall to the ground. Such birds as jays, crows, magpies,
hawks, and other birds of prey, have also been urged as an
exception ; but these are all aggressive birds, able to protect
themselves, and thus do not need any special protection for
their females during nidification. Some birds which build in
covered nests are comparatively dull coloured, like many of
the weaver birds, but in others the colours are more showy,
and in all the sexes are alike ; so that none of these are in any
way opposed to the rule. The golden orioles have, however,
been adduced as a decided exception, since the females are
showy and build in an open nest. But even here the females
are less brilliant than the males, and are sometimes greenish
or olivaceous on the upper surface ; while they very carefully
conceal their nests among dense foliage, and the male is
sufficiently watchful and pugnacious to drive off most in-
truders.

On the other hand, how remarkable it is that the only small
and brightly coloured birds of our own country in which the
male and female are alike — -the' tits and starlings — either
build in holes or construct covered nests ; while the beautiful
hangnests (Icteridpe) of South America, which always build
covered or purse-shaped nests, are equally showy in both
sexes, in striking contrast with the chatterers and tanagers of
the same country, whose females are invariably less conspicuous
than the males. On a rough estimate, there are about 1200
species of birds in the class of showy males and females, with
concealed nidification ; while there are probably, from an
equally rough estimate, about the same number in the con-
trasted class of showy males and dull females, with open
nests. This Avill leave the great bulk of known birds in the
classes of those which are more or less protectively coloured
in both sexes ; or which, from their organisation and habits, do

x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 281

not require special protective coloration, such as many of
the birds of prey, the larger waders, and the oceanic birds.

There are a few very curious cases in which the female
bird is actually more brilliant than the male, and which yet
have open nests. Such are the dotterel (Euclromias morinel-
lus), several species of phalarope, an Australian creeper
(Climacteris erythropus), and a few others ; but in every one
of these cases the relation of the sexes in regard to nidification
is reversed, the male performing the duties of incubation,
while the female is the stronger and more pugnacious. This
curious case, therefore, quite accords with the general law of
coloration.1

Sexual Colours of other Vertebrates.

We may consider a few of the cases of sexual colouring of
other classes of vertebrates, as given by Mr. Darwin. In
fishes, though the sexes are usually alike, there are several
species in which the males are more brightly coloured, and
have more elongated fins, spines, or other appendages, and in
some feAV cases the colours are decidedly different. The males
often fight together, and are altogether more vivacious and
excitable than the females during the breeding season ; and
with this we may connect a greater intensity of coloration.

In frogs and toads the colours are usually alike, or a little
more intense in the males, and the same may be said of most
snakes. It is in lizards that we first meet with considerable
sexual differences, many of the species having gular pouches,
frills, dorsal crests, or horns, either confined to the males, or
more developed in them than in the females, and these orna-
ments are often brightly coloured. In most cases, however,
the tints of lizards are protective, the male being usually a
little more intense in coloration ; and the difference in extreme
cases may be partly due to the need of protection for the
female, which, when laden with eggs, must be less active and
less able to escape from enemies than the male, and may,
therefore, have retained more protective colours, as so many
insects and birds have certainly done.2

In mammalia there is often a somewhat greater intensity

1 Seebohm's History of British Birds, vol. ii. , introduction, p. xiii.
- For details see Darwin's Descent of Man, chap. xii.

282 DARWINISM

of colour in the male, but rarely a decided difference. The
female of the great red kangaroo, however, is a delicate gray ;
while in the Lemur macaco of Madagascar the male is jet-
black and the female brown. In many monkeys also there are
some differences of colour, especially on the face. The sexual
weapons and ornaments of male mammalia, as horns, crests,
manes, and dewlaps, are well known, and are very numerous
and remarkable. Having thus briefly reviewed the facts, we
will now consider the theories to which they have given rise.

Sexual Selection by the Struggles of Males.

Among the higher animals it is a very general fact that
the males fight together for the possession of the females.
This leads, in polygamous animals especially, to the stronger
or better armed males becoming the parents of the next
generation, which inherits the peculiarities of the parents ;
and thus vigour and offensive weapons are continually
increased in the males, resulting in the strength and horns
of the bull, the tusks of the boar, the antlers of the stag,
and the spurs and fighting instinct of the gamecock. But
almost all male animals fight together, though not specially
armed ; even hares, moles, squirrels, and beavers fight to the
death, and are often found to be scarred and wounded. The
same rule applies to almost all male birds ; and these battles
have been observed in such different groups as humming-
birds, finches, goatsuckers, woodpeckers, ducks, and waders.
Among reptiles, battles of the males are known to occur in
the cases of crocodiles, lizards, and tortoises ; among fishes,
in those of salmon and sticklebats. Even among insects the
same law prevails ; and male spiders, beetles of many groups,
crickets, and butterflies often fight together.

From this very general phenomenon there necessarily
results a form of natural selection which increases the vigour
and fighting power of the male animal, since, in every case,
the weaker are either killed, wounded, or driven away. This
selection would be more powerful if males were always in
excess of females, but after much research Mr. Darwin could
not obtain any satisfactory evidence that this was the case.
The same effect, however, is produced in some cases by con-
stitution or habits ; thus male insects usually emerge first from

x COLOURS AND ORNAMENTS CHARACTERISTIC OE SEX 283

the pupa, and among migrating birds the males arrive first
both in this country and in North America. The struggle
is thus intensified, and the most vigorous males are the
first to have offspring. This in all probability is a great
advantage, as the early breeders have the start in securing
food, and the young are strong enough to protect themselves
while the later broods are being produced.

It is to this form of male rivalry that Mr. Darwin first
applied the term " sexual selection." It is evidently a real
power in nature ; and to it we must impute the development of
the exceptional strength, size, and activity of the male, together
with the possession of special offensive and defensive weapons,
and of all other characters which arise from the development
of these or are correlated with them. But he has extended
the principle into a totally different field of action, which
has none of that character of constancy and of inevitable
result that attaches to natural selection, including male
rivalry ; for by far the larger portion of the phenomena,
which he endeavours to explain by the direct action of
sexual selection, can only be so explained on the hypothesis
that the immediate agency is female choice or preference.
It is to this that he imputes the origin of all secondary
sexual characters other than weapons of offence and defence,
of all the ornamental crests and accessory plumes of birds,
the stridulating sounds of insects, the crests and beards
of monkeys and other mammals, and the brilliant colours and
patterns of male birds and butterflies. He even goes further,
and imputes to it a large portion of the brilliant colour that
occurs in both sexes, on the principle that variations occurring
in one sex are sometimes transmitted to the same sex only,
sometimes to both, owing to peculiarities in the laws of inherit-
ance. In this extension of sexual selection to include the
action of female choice or preference, and in the attempt to
give to that choice such wide-reaching effects, I am unable
to follow him more than a very little way ; and I will now
state some of the reasons why I think his views are unsound.

Sexual Characters due to Natural Selection.

Besides the acquisition of weapons by the male for the
purpose of fighting with other males, there are some other

284 DARWINISM

sexual characters which may have been produced by natural
selection. Such are the various sounds and odours which are
peculiar to the male, and which serve as a call to the female
or as an indication of his presence. These are evidently a
valuable addition to the means of recognition of the two sexes,
and are a further indication that the pairing season has
arrived ; and the production, intensification, and differentiation
of these sounds and odours are clearly within the power of
natural selection. The same remark will apply to the peculiar
calls of birds, and even to the singing of the males. These
may well have originated merely as a means of recognition
between the two sexes of a species, and as an invitation from
the male to the female bird. When the individuals of a
species are widely scattered, such a call must be of great
importance in enabling pairing to take place as early as
possible, and thus the clearness, loudness, and individuality of
the song becomes a useful character, and therefore the subject
of natural selection. Such is especially the case with the
cuckoo, and with all solitary birds, and it may have been
equally important at some period of the development of all
birds. The act of singing is evidently a pleasurable one ; and
it probably serves as an outlet for superabundant nervous
energy and excitement, just as dancing, singing, and field
sports do with us. It is suggestive of this view that the
exercise of the vocal power seems to be complementary
to the development of accessory plumes and ornaments,
all our finest singing birds being plainly coloured, and
with no crests, neck or tail plumes to display ; while the
gorgeously ornamented birds of the tropics have no song,
and those which expend much energy in display of plumage,
as the turkey, peacocks, birds of paradise, and' humming-
birds, have comparatively an insignificant development of
voice. Some birds have, in the wings or tail, peculiarly
developed feathers which produce special sounds. In some of
the little manakins of Brazil, two or three of the wing-
feathers are curiously shaped and stiffened in the male, so
that the bird is able to produce with them a pecxiliar
snapping or cracking sound ; and the tail-feathers of several
species of snipe are so narrowed as to produce distinct
drumming, whistling;, or switching sounds when the birds

X COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 285

descend rapidly from a great height. All these are probably
recognition and call notes, useful to each species in relation
to the most important function of their lives, and thus capable
of being developed by the agency of natural selection.

Decorative Plumage of Birds and its Display.

Mr. Darwin has devoted four chapters of his Descent of
Man to the colours of birds, their decorative plumage, and
its display at the pairing season ; and it is on this latter
circumstance that he founds his theory, that both the
plumage and the colours have been developed by the prefer-
ence of the females, the more ornamented males becoming the
parents of each successive generation. Any one who reads
these most interesting chapters will admit, that the fact of the
display is demonstrated ; and it may also be admitted, as highly
probable, that the female is pleased or excited by the display.
But it by no means follows that slight differences in the shape,
pattern, or colours of the ornamental plumes are what lead a
female to give the preference to one male over another ; still
less that all the females of a species, or the great majority of
them, over a wide area of country, and for many successive
generations, prefer exactly the same modification of the colour
or ornament.

The evidence on this matter is very scanty, and in most
cases not at all to the point. Some peahens preferred an old
pied peacock ; albino birds in a state of nature have never
been seen paired with other birds ; a Canada goose paired with
a Bernicle gander ; a male widgeon preferred a pintail duck
to its own species ; a hen canary preferred a male greenfinch
to either linnet, goldfinch, siskin, or chaffinch. These cases
are evidently exceptional, and are not such as generally occur
in nature ; and they only prove that the female does exert
some choice between very different males, and some observa-
tions on birds in a state of nature prove the same thing ; but
there is no evidence that slight variations in the colour or
plumes, in ■ the way of increased intensity or complexity, are
what determines the choice. On the other hand, Mr. Darwin
gives much evidence that it is not so determined. He tells us
that Messrs. Hewitt, Tegetmeier, and Brent, three of the
highest authorities and best observers, " do not believe that

286 DARWINISM

the females prefer certain males on account of the beauty of
their plumage." Mr. Hewitt was convinced " that the female
almost invariably prefers the most vigorous, defiant, and
mettlesome male;" and Mr. Tegetmeier, "that a gamecock,
though disfigured by being dubbed, and with his hackles
trimmed, would bd accepted as readily as a male retaining all
his natural ornaments."1 Evidence is adduced that a female
pigeon will sometimes take an antipathy to a particular male
without any assignable cause ; or, in other cases, will take a
strong fancy to some one bird, and will desert her own mate
for him ; but it is not stated that superiority or inferiority
of plumage has anything to do with these fancies. Two
instances are indeed given, of male birds being rejected, which
had lost their ornamental plumage ; but in both cases (a
widow- finch and a silver pheasant) the long tail-plumes are
the indication of sexual maturity. Such cases do not support
the idea that males with the tail-feathers a trifle longer, or
the colours a trifle brighter, are generally preferred, and
that those which are only a little inferior are as generally
rejected, — and this is what is absolutely needed to establish
the theory of the development of these plumes by means of
the choice of the female.

It will be seen, that female birds have unaccountable likes
and dislikes in the matter of their partners, just as we have
ourselves, and this may afford us an illustration. A young
man, when courting, brushes or curls his hair, and has his
moustache, beard, or whiskers in perfect order, and no doubt
his sweetheart admires them ; but this does not prove that
she marries him on account of these ornaments, still less that
hair, beard, whiskers, and moustache were developed by the
continued preferences of the female sex. So, a girl likes to see
her lover well and fashionably dressed, and he always dresses
as well as he can when he visits her ; but we cannot conclude
from this that the whole series of male costumes, from the
brilliantly coloured, puffed, and slashed doublet and hose of
the Elizabethan period, through the gorgeous coats, long
waistcoats, and pigtails of the early Georgian era, down to
the funereal dress-suit of the present day, are the direct result
of female preference. In like manner, female birds may be
1 Descent of Man, pp. 417, 418, 420.

x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 287

charmed or excited by the fine display of plumage by the
males ; but there is no proof whatever that slight differences
in that display have any effect in determining their choice of
a partner.

Display of Decorative Plumage.

The extraordinary manner in which most birds display
their plumage at the time of courtship, apparently with the
full knowledge that it is beautiful, constitutes one of Mr.
Darwin's strongest arguments. It is, no doubt, a very curious
and interesting phenomenon, and indicates a connection be-
tween the exertion of particular muscles and the develop-
ment of colour and ornament ; but, for the reasons just given,
it does not prove that the ornament has been developed by
female choice. During excitement, and when the organism
develops superabundant energy, many animals find it pleasur-
able to exercise their various muscles, often in fantastic ways,
as seen in the gambols of kittens, lambs, and other young
animals. But at the time of pairing, male birds are in a
state of the most perfect development, and possess an
enormous store of vitality ; and under the excitement of the
sexual passion they perform strange antics or rapid flights, as
much probably from an internal impulse to motion and exertion
as with any desire to please their mates. Such are the rapid
descent of the snipe, the soaring and singing of the lark, and
the dances of the cock-of-the-rock and of many other birds.

It is very suggestive that similar strange movements are
performed by many birds which have no ornamental plumage
to display. Goatsuckers, geese, carrion vultures, and many
other birds of plain plumage have been observed to dance,
spread their wings or tails, and perform strange love-antics.
The courtship of the great albatross, a most unwieldy and
dull coloured bird, has been thus described by Professor
Moseley : " The male, standing by the female on the nest,
raises his wings, spreads his tail and elevates it, throws up his
head with the bill in the air, or stretches it straight out, or
forwards, as far as he can, and then utters a curious cry."1 Mr.
Jenner Weir informs me that " the male blackbird is full of
action, spreads out his glossy wing and tail, turns his rich golden

1 Notes of a Naturalist on the Challenger.

288 DARWINISM chap.

beak towards the female, and chuckles with delight," while he
has never seen the more plain coloured thrush demonstrative
to the female. The linnet distends his rosy breast, and
slightly expands his brown wings and tail ; while the various
gay coloured Australian finches adopt such attitudes and
postures as, in every case, to show off their variously coloured
plumage to the best advantage.1

A Theory of Animal Coloration.

Having rejected Mr. Darwin's theory of female choice as
incompetent to account for the brilliant colours and markings
of the higher animals, the preponderance of these colours and
markings in the male sex, and their display during periods
of activity or excitement, I may be asked what explanation
I have to offer as a preferable substitute. In my Tropical
Nature I have already indicated such a theory, which I will
now briefly explain, supporting it by some additional facts
and arguments, which appear to me to have great weight, and
for which I am mainly indebted to a most interesting and
suggestive posthumous work by Mr. Alfred Tylor.2

The fundamental or ground colours of animals are, as has
been shown in preceding chapters, very largely protective,
and it is not improbable that the primitive colours of all
animals were so. During the long course of animal develop-
ment other modes of protection than concealment by harmony
of colour arose, and thenceforth the normal development of
colour due to the complex chemical and structural changes
ever going on in the organism, had full play ; and the colours
thus produced were again and again modified by natural selection
for purposes of warning, recognition, mimicry, or special pro-
tection, as has been already fully explained in the preceding
chapters.

Mr. Tylor has, however, called attention to an important
principle which underlies the various patterns or ornamental
markings of animals — namely, that diversified coloration
follows the chief lines of structure, and changes at points, such
as the joints, where function changes. He says, "If Ave
take highly decorated species — that is, animals marked by

1 Descent of Man, pp. 401, 402.
3 Coloration in Animals and Plants, London, 18SG.

x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 289

alternate dark or light bands or spots, such as the zebra, some
deer, or the carnivora, we find, first, that the region of the
spinal column is marked by a dark stripe ; secondly, that the
regions of the appendages, or limbs, are differently marked ;
thirdly, that the flanks are striped or spotted, along or be-
tween the regions of the lines of the ribs ; fourthly, that the
shoulder and hip regions are marked by curved lines ; fifthly,
that the pattern changes, and the direction of the lines, or
spots, at the head, neck, and every joint of the limbs ; and
lastly, that the tips of the ears, nose, tail, and feet, and the
eye are emphasised in colour. In spotted animals the greatest
length of the spot is generally in the direction of the largest
development of the skeleton."

This structural decoration is well seen in many insects. In
caterpillars, similar spots and markings are repeated in each
segment, except where modified for some form of protection.
In butterflies, the spots and bands usually have reference to
the form of the wing and the arrangement of the nervures ;
and there is much evidence to show that the primitive mark-
ings are always spots in the cells, or between the nervures, or
at the junctions of nervures, the extension and coalescence of
these spots forming borders, bands, or blotches, which have
become modified in infinitely varied ways for protection,
warning, or recognition. Even in birds, the distribution of
colours and markings follows generally the same law. The
crown of the head, the throat, the ear-coverts, and the eyes
have usually distinct tints in all highly coloured birds • the
region of the furcula has often a distinct patch of colour,
as have the pectoral muscles, the uropygium or root of the tail,
and the under tail-coverts.1

Mr. Tylor was of opinion that the primitive form of
ornamentation consisted of spots, the confluence of these in
certain directions forminglines or bands; and, these again, some-
times coalescing into blotches, or into more or less uniform
tints covering a large portion of the surface of the body. The
young lion and tiger are both spotted ; and in the Java hog
(Sus vittatus) very young animals are banded, but have spots
over the shoulders and thighs. These spots run into stripes

1 Coloration of Animals, PI. X, p. 90 ; and Pis. II, III, and IV, pp. 30,
40, 42.

U

290 DARWINISM chap.

as the animal grows older; then the stripes expand, and
at last, meeting together, the adult animal becomes of a
uniform dark brown colour. So many of the species of
deer are spotted Avhen young, that Darwin concludes the
ancestral form, from which all deer are derived, must have
been spotted. Pigs and tapirs are banded or spotted when
young ; an imported young specimen of Tapirus Bairdi
was covered with white spots in longitudinal rows, here
and there forming short stripes.1 Even the horse, which
Darwin supposes to be descended from a striped animal,
is often spotted, as in dappled horses ; and great numbers
show a tendency to spottiness, esj)ecially on the haunches.

Ocelli may also be developed from spots, or from bars, as
pointed out by Mr. Darwin. Spots are an ordinary form of
marking in disease, and these spots sometimes run together,
forming blotches. There is evidence that colour markings are
in some way dependent on nerve distribution. In the disease
known as frontal herpes, an eruption occurs which corresponds
exactly to the distribution of the ophthalmic division of the
fifth cranial nerve, mapping out all its little branches even
to the one which goes to the tip of the nose. In a Hindoo
suffering from herpes the pigment was destroyed in the arm
along the course of the ulnar nerve, with its branches along
both sides of one finger and the half of another. In the leg
the sciatic and scaphenous nerves were partly mapped out,
giving to the patient the appearance of an anatomical
diagram.2

These facts are very interesting, because they help to
explain the general dependence of marking on structure Avhich
has been already pointed out. For, as the nerves everywhere
follow the muscles, and these are attached to the various bones,
we see how it happens, that the tracts in which distinct
developments of colour appear, should so often be marked out
by the chief divisions of the bony structure in vertebrates, and
by the segments in the annulosa. There is, however, another
correspondence of even greater interest and importance.
Brilliant colours usually appear just in proportion to the

1 See coloured Fig. in Proc. Zool. Soc, 1871, p. 626.

2 A. Tylor's Coloration, p. 40 ; and Photograph in Hutchinson's Illustra-
tions of Clinical Surgery, quoted by Tylor.

x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 291

development of tegumentary appendages. Among birds the
most brilliant colours are possessed by those which have
developed frills, crests, and elongated tails like the humming-
birds; immense tail -coverts like the peacock; enormously
expanded wing-feathers, as in the argus-pheasant ; or magnifi-
cent plumes from the region of the coracoids in many of
the birds of paradise. It is to be noted, also, that all these
accessory plumes spring from parts of the body which, in
other species, are distinguished by patches of colour ; so that
we may probably impute the development of colour and of
accessory plumage to the same fundamental cause.

Among insects, the most brilliant and varied coloration
occurs in the butterflies and moths, groups in which the wing-
membranes have received their greatest expansion, and whose
specialisation has been carried furthest in the marvellous scaly
covering which is the seat of the colour. It is suggestive, that
the only other group in which functional wings are much
coloured is that of the dragonflies, where the membrane is
exceedingly expanded. In like manner, the colours of beetles,
though greatly inferior to those of the lepidoptera, occur in a
group in which the anterior pair of wings has been thickened
and modified in order to protect the vital parts, and in which
these wing-covers (elytra), in the course of development in the
different groups, must have undergone great changes, and have
been the seat of very active growth.

The Origin of Accessory Plumes.

Mr. Darwin supposes, that these have in almost every case
been developed by the preference of female birds for such
males as possessed them in a higher degree than others ; but
this theory does not account for the fact that these plumes
usually appear in a few definite parts of the body. We
require some cause to initiate the development in one part
rather than in another. Noav, the view that colour has arisen
over surfaces where muscular and nervous development is
considerable, and the fact that it appears especially upon the
accessory or highly developed plumes, leads us to inquire whether
the same cause has not primarily determined the development
of these plumes. The immense tuft of golden plumage in the
best known birds of paradise (Paradisea apoda and P. minor)

292 DARWINISM

springs from a very small area on the side of the breast. Mr.
Frank E. Beddard, who has kindly examined a specimen for
me, says that "this area lies upon the pectoral muscles, and
near to the point where the fibres of the muscle converge
towards their attachment to the humerus. The plumes arise,
therefore, close to the most powerful muscle of the body, and
near to where the activities of that muscle would be at a
maximum. Furthermore, the area of attachment of the plumes
is just above the point where the arteries and nerves for the
supply of the pectoral muscles, and neighbouring regions,
leave the interior of the body. The area of attachment of
the plume is, also, as you say in your letter, just above the
junction of the coracoid and sternum." Ornamental plumes
of considerable size rise from the same part in many other
species of paradise birds, sometimes extending laterally in front,
so as to form breast shields. They also occur in many humming-
birds, and in some sun-birds and honey-suckers; and in all these
cases there is a wonderful amount of activity and rapid move-
ment, indicating a surplus of vitality, which is able to manifest
itself in the development of these accessory plumes.1

In a quite distinct set of birds, the gallinacese, we find the
ornamental plumage usually arising from very different parts, in
the form of elongated tail-feathers or tail-coverts, and of ruffs
or hackles from the neck. Here the wings are comparatively
little used, the most constant activities depending on the legs,
since the gallinacese are pre-eminently walking, running, and
scratching birds. Now the magnificent train of the peacock
— the grandest development of accessory plumes in this order
— springs from an oval or circular area, about three inches in
diameter, just above the base of the tail, and, therefore,
situated over the lower part of the spinal column near the
insertion of the powerful muscles which move the hind limbs
and elevate the tail. The very frequent presence of neck-ruffs
or breast-shields in the males of birds with accessory plumes
may be partly clue to selection, because they must serve as a
protection in their mutual combats, just as does the lion's or the
horse's mane. The enormously lengthened plumes of the bird
of paradise and of the peacock can, however, have no such use,

1 For activity and pugnacity of humming-birds, see Trojrical Nature, pp.
130, 213.

x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 293

but must be rather injurious than beneficial in the bird's ordi-
nary life. The fact that they have been developed to so great
an extent in a few species is an indication of such perfect adapta-
tion to the conditions of existence, such complete success in
the battle for life, that there is, in the adult male at all events,
a surplus of strength, vitality, and growth-power which is able
to expend itself in this way without injury. That such is the
case is shown by the great abundance of most of the species
which possess these wonderful superfluities of plumage. Birds
of paradise are among the commonest birds in New Guinea,
and their loud voices can be often heard when the birds them-
selves are invisible in the depths of the forest ; while Indian
sportsmen have described the peafowl as being so abundant,
that from twelve to fifteen hundred have been seen within
an hour at one spot ; and they range over the whole country
from the , Himalayas to Ceylon. Why, in allied species, the
development of accessory plumes has taken different forms, we
are unable to say, except that it may be due to that individual
variability which has served as the starting-point for so much
of what seems to us strange in form, or fantastic in colour,
both in the animal and vegetable world.

Development of Accessory Plumes and their Display.

If we have found a vera causa for the origin of ornamental
appendages of birds and other animals in a surplus of vital
energy, leading to abnormal growths in those parts of the
integument where muscular and nervous action are greatest,
the continuous development of these appendages will result
from the ordinary action of natural selection in preserving the
most healthy and vigorous individuals, and the still further
selective agency of sexual struggle in giving to the very
strongest and most energetic the parentage of the next genera-
tion. And, as all the evidence goes to show that, so far as
female birds exercise any choice, it is of "the most vigorous,
defiant, and mettlesome male," this form of sexual selection
will act in the same direction, and help to carry on the process
of plume development to its culmination. That culmination
will be reached when the excessive length or abundance of the
plumes begins to be injurious to the bearer of them ; and it
may be this check to the further lengthening of the peacock's

294 DARWINISM

train that lias led to the broadening of the feathers at the
ends, and the consequent production of the magnificent eye-
spots which now form its crowning ornament.

The display of these plumes will result from the same
causes which led to their production. Just in proportion as
the feathers themselves increased in length and abundance, the
skin-muscles which serve to elevate them would increase also ;
and the nervous development as well as the supply of blood
to these parts being at a maximum, the erection of the plumes
would become a habit at all periods of nervous or sexual
excitement. The display of the plumes, like the existence of
the plumes themselves, would be the chief external indication
of the maturity and vigour of the male, and would, therefore,
be necessarily attractive to the female. We have, thus, no
reason for imputing to her any of those aesthetic emotions
which are excited in us, by the beauty of form, colour, and
pattern of these plumes ; or the still more improbable sesthetic
tastes, which would cause her to choose her mate on account
of minute differences in their forms, colours, or patterns.

As co-operating causes in the production of accessory
ornamental plumes, I have elsewhere suggested l that crests
and other erectile feathers may have been useful in making
the bird more formidable in appearance, and thus serving to
frighten away enemies ; while long tail or wing feathers might
serve to distract the aim of a bird of prey. But though this
might be of some use in the earlier stages of their develop-
ment, it is probably of little importance compared with the
vigour and pugnacity of which the plumes are the indication,
and which enable most of their possessors to defend them-
selves against the enemies which are dangerous to weaker
and more timid birds. Even the tiny humming-birds are said
to attack birds of prey that approach too near to their nests.

The Effect of Female Preference will be Neutralised by
Natural Selection.

The various facts and arguments now briefly set forth,
afford an explanation of the phenomena of male ornament,

1 Tropical Nature, p. 209. In Chapter V of this work the views here
advocated were first set forth, and the reader is referred there for further
details.

x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 295

as being due to the general laws of growth and develop-
ment, and make it unnecessary to call to our aid so hypo-
thetical a cause as the cumulative action of female prefer-
ence. There remains, however, a general argument, arising
from the action of natural selection itself, which renders it
almost inconceivable that female preference could have been
effective in the way suggested ; while the same argument
strongly supports the view here set forth. Natural selec-
tion, as we have seen in our earlier chapters, acts per-
petually and on an enormous scale in weeding out the
"unfit" at every stage of existence, and preserving only
those which are in all respects the very best. Each year, only
a small percentage of young birds survive to take the place of
the old birds which die ; and the survivors will be those which
are best able to maintain existence from the egg onwards, an
important factor being that their parents should be well able
to feed and protect them, while they themselves must in turn
be equally able to feed and protect their own offspring. Now
this extremely rigid action of natural selection must render
any attempt to select mere ornament utterly nugatory, unless
the most ornamented always coincide with "the fittest "in
every other respect ; while, if they do so coincide, then any
selection of ornament is altogether superfluous. If the most
brightly coloured and fullest plumaged males are not the most
healthy and vigorous, have not the best instincts for the proper
construction and concealment of the nest, and for the care
and protection of the young, they are certainly not the fittest,
and will not survive, or be the parents of survivors. If, on
the other hand, there is generally this correlation — if, as has
been here argued, ornament is the natural product and direct
outcome of superabundant health and vigour, then no other
mode of selection is needed to account for the presence of
such ornament. The action of natural selection does not
indeed disprove the existence of female selection of ornament,
as ornament, but it renders it entirely ineffective ; and as
the direct evidence for any such female selection is almost
nil, while the objections to it are certainly weighty, there can
be no longer any reason for upholding a theory which was
provisionally useful in calling attention to a most curious and
suggestive body of facts, but which is now no longer tenable.

296 DARWINISM

The term " sexual selection " must, therefore, be restricted
to the direct results of male struggle and combat. This is
really a form of natural selection, and is a matter of direct
observation ; Avhile its results are as clearly deducible as those
of any of the other modes in which selection acts. And if
this restriction of -the term is needful in the case of the higher
animals it is much more so with the lower. In butterflies the
weeding out by natural selection takes place to an enormous
extent in the egg, larva, and pupa states ; and perhaps not
more than one in a hundred of the eggs laid produces a perfect
insect which lives to breed. Here, then, the impotence of
female selection, if it exist, must be complete ; for, unless the
most brilliantly coloured males are those which produce the
best protected eggs, larvae, and pupge, and unless the particular
eggs, larvse, and pupa?, which are able to survive, are those
which produce the most brilliantly coloured butterflies, any
choice the female might make must be completely swamped.
If, on the other hand, there is this correlation between colour
development and perfect adaptation to conditions in all stages,
then this development will necessarily proceed by the agency
of natural selection and the general laws which determine
the production of colour and of ornamental appendages.1

General Laws of Animal Coloration.
The condensed account which has now been given of the
phenomena of colour in the animal world will sufficiently show
the wonderful complexity and extreme interest of the subject ;
while it affords an admirable illustration of the importance of
the great principle of utility, and of the effect of the theories
of natural selection and development in giving a new interest

1 The Rev. 0. Pickard-Cambridge, who has devoted himself to the study
of spiders, has kindly sent me the following extract from a letter, written
in 1869, in which he states his views on this question : —

"I myself doubt that particular application of the Darwinian theory
which attributes male peculiarities of form, structure, colour, and ornament
to female appetency or predilection. There is, it seems to me, undoubtedly
something in the male organisation of a special, and sexual nature, which,
of its own vital force, develops the remarkable male peculiarities so
commonly seen, and of no imaginable use to that sex. In as far as
these peculiarities show a great vital power, they point out to its the finest
and strongest individuals of the sex, and show us which of them would
most certainly appropriate to themselves the best and greatest number of
females, and leave behind them the strongest and greatest number of

x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 297

to the most familiar facts of nature. Much yet remains to be
done, both in the observation of new facts as to the relations
between the colours of animals and their habits or economy,
and, more especially, in the elucidation of the laws of growth
which determine changes of colour in the various groups ; but
so much is already known that we are able, with some
confidence, to formulate the general principles which have
brought about all the beauty and variety of colour which
everywhere delight us in our contemplation of animated
nature. A brief statement of these principles will fitly con-
clude our exposition of the subject.

1 . Colour may be looked upon as a necessary result of the
highly complex chemical constitution of animal tissues and
fluids. The blood, the bile, the bones, the fat, and other
tissues have characteristic, and often brilliant colours, which
we cannot suppose to have been determined for any special
purpose, as colours, since they are usually concealed. The
external organs, with their various appendages and integu-
ments, would, by the same general laws, naturally give rise to
a greater variety of colour.

2. We find it to be the fact that colour increases in variety
and intensity as external structures and dermal appendages
become more differentiated and developed. It is on scales,
hair, and especially on the more highly specialised feathers,
that colour is most varied and beautiful ; while among insects
colour is most fully developed in those whose wing membranes
are most expanded, and, as in the lepicloptera, are clothed
with highly specialised scales. Here, too, we find an additional
mode of colour production in transparent lamellae or in fine
surface striae which, by the laws of interference, produce the
wonderful metallic hues of so many birds and insects.

progeny. And here would come in, as it appears to me, the proper
application of Darwin's theory of Natural Selection ; for the possessors
of greatest vital power being those most frequently produced and repro-
duced, the external signs of it would go on developing in an ever-increasing
exaggeration, only to be checked where it became really detrimental in some
respect or other to the individual."

This passage, giving the independent views of a close observer — one,
moreover, who has studied the species of an extensive groivp of animals
both in the field and in the laboratory — very nearly accords with my own
conclusions above given ; and, so far as the matured opinions of a competent
naturalist have any weight, afford them an important support.

298 DARWINISM

3. There are indications of a progressive change of colour,
perhaps in some definite order, accompanying the development
of tissues or appendages. Thus spots spread and fuse into
bands, and when a lateral or centrifugal expansion has
occurred — as in the termination of the peacocks' train feathers,
the outer web of the secondary quills of the Argus pheasant,
or the broad and rounded wings of many butterflies — into
variously shaded or coloured ocelli. The fact that we find
gradations of colour in many of the more extensive groups,
from comparatively dull or simple to brilliant and varied hues,
is an indication of some such law of development, due
probably to progressive local segregation in the tissues of
identical chemical or organic molecules, and dependent on
laws of growth yet to be investigated.

4. The colours thus produced, and subject to much in-
dividual variation, have been modified in innumerable ways
for the benefit of each species. The most general modifica-
tion has been in such directions as to favour concealment
when at rest in the usual surroundings of the species, some-
times carried on by successive steps till it has resulted in the
most minute imitation of some inanimate object or exact
mimicry of some other animal. In other cases bright colours
or striking contrasts have been preserved, to serve as a warning
of inedibility or of dangerous powers of attack. Most frequent
of all has been the specialisation of each distinct form by some
tint or marking for purposes of easy recognition, especially in
the case of gregarious animals whose safety largely depends
upon association and mutual defence.

5. As a general rule the colours of the two sexes are alike;
but in the higher animals there appears a tendency to deeper
or more intense colouring in the male, due probably to his
greater vigour and excitability. In many groups in which
this superabundant vitality is at a maximum, the develop-
ment of dermal appendages and brilliant colours has gone on
increasing till it has resulted in a great diversity between the
sexes ; and in most of these cases there is evidence to show
that natural selection has caused the female to retain the
primitive and more sober colours of the group for purposes of
protection.

x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 299

Concluding Remarks.

The general principles of colour development now sketched
out enable us to give some rational explanation of the
wonderful amount of brilliant colour which occurs among
tropical animals. Looking on colour as a normal product of
organisation, which has either been allowed free play, or has
been checked and modified for the benefit of the species, we
can see at once that the luxuriant and perennial vegetation of
the tropics, by affording much more constant means of con-
cealment, has rendered brilliant colour less hurtful there than
in the temperate and colder regions. Again, this perennial
vegetation supplies abundance of both vegetable and insect
food throughout the year, and thus a greater abundance and
greater variety of the forms of life are rendered possible, than
where recurrent seasons of cold and scarcity reduce the
possibilities of life to a minimum. Geology furnishes us with
another reason, in the fact, that throughout the tertiary period
tropical conditions prevailed far into the temperate regions, so
that the possibilities of colour development were still greater
than they are at the present time. The tropics, therefore,
present to us the results of animal development in a much
larger area and under more favourable conditions than
prevail to-day. We see in them samples of the productions of
an earlier and a better world, from an animal point of view ;
and this probably gives a greater variety and a finer display of
colour than would have been produced, had conditions always
been what they are now. The temperate zones, on the other
hand, have recently suffered the effects of a glacial period of
extreme severity, with the result that almost the only gay
coloured birds they now possess are summer visitors from
tropical or sub -tropical lands. It is to the unbroken and
almost unchecked course of development from remote geo-
logical times that has prevailed in the tropics, favoured by
abundant food and perennial shelter, that we owe such superb
developments as the frills and crests and jewelled shields of
the humming-birds, the golden plumes of the birds of paradise,
and the resplendent train of the peacock. This last exhibits to
us the culmination of that marvel and mystery of animal colour
which is so well expressed by a poet-artist in the following

300 DARWINISM

lines. The marvel will ever remain to the sympathetic
student of nature, but I venture to hope that in the pre-
ceding chapters I have succeeded in lifting — if only by one
of its corners — the veil of mystery which has for long
shrouded this department of nature.

On a Peacock's Feather.

In Nature's workshop but a shaving,

Of her poem but a word,
But a tint brushed from her palette,

This feather of a bird !
Yet set it in the sun glance,

Display it in the shine,
Take graver's lens, explore it,

Note filament and line,
Mark amethyst to sapphire,

And sapphire to gold,
And gold to emerald changing

The archetype unfold !
Tone, tint, thread, tissue, texture,

Through every atom scan,
Conforming still, developing,

Obedient to plan.
This but to form a pattern

On the garment of a bird !
What then must be the poem,

This but its lightest word !
Sit before it ; ponder o'er it,

'Twill thy mind advantage more,
Than a treatise, than a sermon,

Than a library of lore.

CHAPTER XI

THE SPECIAL COLOURS OF PLANTS : THEIR ORIGIN
AND PURPOSE

The general colour relations of plants — Colours of fruits — The meaning of
nuts — Edible or attractive fruits — The colours of flowers — Modes of
securing cross-fertilisation — The interpretation of the facts — Summary
of additional facts bearing on insect fertilisation — Fertilisation of
flowers by birds — Self-fertilisation of flowers — Difficulties and con-
tradictions— Intercrossing not necessarily advantageous — Supposed
evil results of close interbreeding — How the struggle for existence
acts among flowers — Flowers the product of insect agency — Concluding
remarks on colour in nature.

The colours of plants are both less definite and less complex
than are those of animals, and their interpretation on the
principle of utility is, on the whole, more direct and more
easy. Yet here, too, we find that in our investigation of the
uses of the various colours of fruits and flowers, we are
introduced to some of the most obscure recesses of nature's
workshop, and are confronted with problems of the deepest
interest and of the utmost complexity.

So much has been written on this interesting subject
since Mr. Darwin first called attention to it, and its main
facts have become so generally known by means of lectures,
articles, and popular books, that I shall give here a mere
outline sketch, for the purpose of leading up to a discussion
of some of the more fundamental problems which arise out of
the facts, and which have hitherto received less attention than
they deserve.

302 DARWINISM

The General Colour Relations of Plants.

The green colour of the foliage of leafy plants is due to
the existence of a substance called chlorophyll, which is
almost universally . developed in the leaves under the action
of light. It is subject to definite chemical changes during
the processes of growth and of decay, and it is owing to
these changes that we have the delicate tints of spring
foliage, and the more varied, intense, and gorgeous hues of
autumn. But these all belong to the class of intrinsic or
normal colours, due to the chemical constitution of the
organism ; as colours they are unadaptive, and appear to
have no more relation to the wellbeing of the plants them-
selves than do the colours of gems and minerals. We may
also include in the same category those algse and fungi
which have bright colours — the "red snow" of the arctic
regions, the reel, green, or purple seaweeds, the brilliant
scarlet, yellow, white, or black agarics, and other fungi.
All these colours are probably the direct results of chemical
composition or molecular structure, and, being thus normal
products of the vegetable organism, need no special explana-
tion from our present point of view ; and the same remark
will apply to the varied tints of the bark of trunks, branches,
and twigs, which are often of various shades of brown and
green, or even vivid reds or yellows.

There are, however, a few cases in which the need of
protection, which we have found to be so important an
agency in modifying the colours of animals, has also deter-
mined those of some of the smaller members of the vegetable
kingdom. Dr. Burchell found a mesembryanthemum in
South Africa like a curiously shaped pebble, closely resem-
bling the stones among which it grew;1 and Mr. J. P. Mansel
AVeale states that in the same country one of the Asclepi-
adea? has tubers growing above ground among stones which
they exactly resemble, and that, when not in leaf, they
are for this reason quite invisible.2 It is clear that such
resemblances must be highly useful to these plants, inhabiting
an arid country abounding in herbivorous mammalia, which,

1 Burchell's Travels, vol. i. p. 10.
- Nature, vol. iii. p. 507.

xi THE SPECIAL COLOURS OF PLANTS 303

in times of drought or scarcity, will devour everything in the
shape of a fleshy stem or tuber.

True mimicry is very rare in plants, though adaptation to
like conditions often produces in foliage and habit a similarity
that is deceiving. Euphorbias growing in deserts often closely
resemble cacti. Seaside plants and high alpine plants of
different orders are often much alike ; and innumerable
resemblances of this kind are recorded in the names of
plants, as Veronica epacridea (the veronica like an epacris),
Limnanthemum nymphseoicles (the limnanthemum like a
nymphsea), the resembling species in each case belonging to
totally distinct families. But in these cases, and in most others
that have been observed, the essential features of true mimicry
are absent, inasmuch as the one plant cannot be supposed to
derive any benefit from its close resemblance to the other,
and this is still more certain from the fact that the two
species usually inhabit different localities. A few cases exist,
however, in which there does seem to be the necessary
accordance and utility. Mr. Mansel Weale mentions a labiate
plant (Ajuga ophrydis), the only species of the genus Ajuga in
South Africa, which is strikingly like an orchid of the same
country ; while a balsam (Impatiens capensis), also a solitary
species of the genus in that country, is equally like an orchid,
growing in the same locality and visited by the same insects.
As both these genera of plants are specialised for insect
fertilisation, and both of the plants in question are isolated
species of their respective genera, we may suppose that,
when they first reached South Africa they were neglected
by the insects of the country ; but, being both remotely like
orchids in form of flower, those varieties that approached
nearest to the familiar species of the country were visited
by insects and cross-fertilised, and thus a closer resemblance
would at length be brought about. Another case of close
general resemblance, is that of our common white dead-
nettle (Lamium album) to the stinging-nettle (Urtica dioica) ■
and Sir John Lubbock thinks that this is a case of true
mimicry, the dead-nettle being benefited by being mistaken
by grazing animals for the stinging-nettle.1

1 Flowers, Fruits, and Leaves, p. 128 (Fig. 79).

304 DARWINISM

Colours of Fruits.

It is when we come to the essential parts of plants on
which their perpetuation and distribution depends, that we
find colour largely utilised for a distinct purpose in flowers
and fruits. In the former we find attractive colours and
guiding marks to secure cross - fertilisation by insects ; in
the latter attractive or protective coloration, the first to
attract birds or other animals when the fruits are intended to be
eaten, the second to enable them to escape being eaten when
it would be injurious to the species. The colour phenomena
of fruits being much the most simple will be considered first.

The perpetuation and therefore the very existence of each
species of flowering plant depend upon its seeds being pre-
served from destruction and more or less effectually dispersed
over a considerable area. The dispersal is effected either
mechanically or by the agency of animals. Mechanical dis-
persal is chiefly by means of air-currents, and large numbers
of seeds are specially adapted to be so carried, either by being
clothed with down or pappus, as in the well-known thistle and
dandelion seeds ; by having wings or other appendages, as in
the sycamore, birch, and many other trees ; by being thrown
to a considerable distance by the splitting of the seed-vessel,
and by many other curious devices.1 Very large numbers of
seeds, however, are so small and light that they can be carried
enormous distances by gales of wind, more especially as most
of this kind are flattened or curved, so as to expose a large
surface in proportion to their weight. Those which are
carried by animals have their surfaces, or that of the seed-
vessel, armed with minute hooks, or some prickly covering
which attaches itself to the hair of mammalia or the feathers
of birds, as in the burdock, cleavers, and many other species.
Others again are sticky, as in Plumbago europsea, mistletoe,
and many foreign plants.

All the seeds or seed-vessels which are adapted to be
dispersed in any of these ways are of dull protective tints, so
that when they fall on the ground they are almost indis-
tinguishable ; besides which, they are usually small, hard, and

1 For a popular sketch of these, see Sir J. Lubbock's Floicers, Fruits, and
Leaves, or any general botanical work.

xi THE SPECIAL COLOURS OF PLANTS 305

altogether unattractive, never having any soft, juicy pulp ;
while the edible seeds often bear such a small proportion
to the hard, dry envelopes or appendages, that few animals
would care to eat them.

The Meaning of Nuts.

There is, however, another class of fruits or seeds, usually
termed nuts, in which there is a large amount of edible matter,
often very agreeable to the taste, and especially attractive
and nourishing to a large number of animals. But when
eaten, the seed is destroyed and the existence of the species
endangered. It is evident, therefore, that it is by a kind of
accident that these nuts are eatable ; and that they are not
intended to be eaten is shown by the special care nature seems
to have taken to conceal or to protect them. We see that all
our common nuts are green when on the tree, so as not easily
to be distinguished from the leaves ; but when ripe they turn
brown, so that when they fall on to the ground they are equally
indistinguishable among the dead leaves and twigs, or on the
brown earth. Then they are almost always protected by hard
coverings, as in hazel-nuts, which are concealed by the enlarged
leafy involucre, and in the large tropical brazil-nuts and cocoa-
nuts by such a hard and tough case as to be safe from almost
every animal. Others have an external bitter rind, as in the
walnut; while in the chestnuts and beechnuts two or three
fruits are enclosed in a prickly involucre.

Notwithstanding all these precautions, nuts are largely
devoured by mammalia and birds ; but as they are chiefly
the product of trees or shrubs of considerable longevity,
and are generally produced in great profusion, the perpetua-
tion of the species is not endangered. In some cases the
devourers of nuts may aid in their dispersal, as they probably
now and then swallow the seed whole, or not sufficiently
crushed to prevent germination ; while squirrels have been
observed to bury nuts, many of which are forgotten and
afterwards grow in places they could not have otherwise
reached.1 Nuts, especially the larger kinds which are so
well protected by their hard, nearly globular cases, have their
dispersal facilitated by rolling down hill, and more especially

1 Nature, vol. xv. p. 117.
X

306 DARWINISM

by floating in rivers and lakes, and thus reaching other locali-
ties. During the elevation of land areas this method would
be very effective, as the new land would always be at a lower
level than that already covered with vegetation, and therefore
in the best position for being stocked with plants from it.

The other modes of dispersal of seeds are so clearly adapted
to their special wants, that we feel sure they must have been
acquired by the process of variation and natural selection.
The hooked and sticky seeds are always those of such her-
baceous plants as are likely, from their size, to come in
contact with the wool of sheep or the hair of cattle ; while
seeds of this kind never occur on forest trees, on aquatic
plants, or even on very dwarf creepers or trailers. The
winged seed-vessels or seeds, on the other hand, mostly belong
to trees and to tall shrubs or climbers. We have, therefore, a
very exact adaptation to conditions in these different modes of
dispersal ; while, when Ave come to consider individual cases,
we find innumerable other adaptations, some of which the
reader will find described in the little work by Sir John
Lubbock already referred to.

Edible or Attractive Fruits.

It is, however, when we come to true fruits (in a popular
sense) that Ave find varied colours evidently intended to
attract animals, in order that the fruits may be eaten, AAmile
the seeds pass through the body undigested and are then in
the fittest state for germination. This end has been gained in
a great variety of Avays, and A\dth so many corresponding
adaptations as to leave no doubt as to the value of the result.
Fruits are pulpy or juicy, and usually sweet, and form the
favourite food of innumerable birds and some mammals. They
are always coloured so as to contrast with the foliage or
surroundings, red being the most common as it is certainly the
most conspicuous colour, but yellow, purple, black, or white
being not uncommon. The edible portion of fruits is developed
from different parts of the floral envelopes, or of the ovary, in
the various orders and genera. Sometimes the calyx becomes
enlarged and fleshy, as in the apple and pear tribe; more
often the integuments of the ovary itself are enlarged, as in
the plum, peach, grape, etc. ; the receptacle is enlarged and

xi THE SPECIAL COLOURS OF PLANTS 307

forms the fruit of the strawberry ; while the mulberry, pine-
apple, and fig are examples of compound fruits formed in
various ways from a dense mass of flowers.

In all cases the seeds themselves are protected from injury
by various devices. They are small and hard in the straw-
berry, raspberry, currant, etc., and are readily swallowed
among the copious pulp. In the grape they are hard and
bitter ; in the rose (hip) disagreeably hairy ; in the orange
tribe very bitter ; and all these have a smooth, glutinous
exterior which facilitates their being sAvallowed. When the
seeds are larger and are eatable, they are enclosed in an
excessively hard and thick covering, as in the various kinds
of " stone " fruit (plums, peaches, etc.), or in a very tough core,
as in the apple. In the nutmeg of the Eastern Archipelago
we have a curious adaptation to a single group of birds. The
fruit is yellow, somewhat like an oval peach, but firm and
hardly eatable. This splits open and shows the glossy
black covering of the seed or nutmeg, over which spreads
the bright scarlet arillus or "mace," an adventitious growth
of no use to the plant except to attract attention. Large
fruit pigeons pluck out this seed and swallow it entire
for the sake of the mace, while the large nutmeg passes
through their bodies and germinates ; and this has led to
the wide distribution of wild nutmegs over New Guinea
and the surrounding islands.

In the restriction of bright colour to those edible fruits the
eating of which is beneficial to the plant, we see the undoubted
result of natural selection ; and this is the more evident when
we find that the colour never appears till the fruit is ripe —
that is, till the seeds within it are fully matured and in the
best state for germination. Some brilliantly coloured fruits
are poisonous, as in our bitter-sweet (Solanum dulcamara),
cuckoo-pint (Arum) and the West Indian manchineel. Many
of these are, no doubt, eaten by animals to whom they are
harmless ; and it has been suggested that even if some
animals are poisoned by them the plant is benefited, since it
not only gets dispersed, but finds, in the decaying body
of its victim, a rich manure heap.1 The particular colours
of fruits are not, so far as we know, of any use to them other
-1 Grant Allen's Colour Sense, p. 113.

308 DARWINISM

than as regards conspicuousness, hence a tendency to any
decided colour has been preserved and accumulated as serving
to render the fruit easily visible among its surroundings of
leaves or herbage. Out of 134 fruit- bearing plants in
Mongredien's Trees and Shrubs, and Hooker's British Flora,
the fruits of no less than sixty-eight, or rather more than half,
are red, forty-five are black, fourteen yellow, and seven white.
The great prevalence of red fruits is almost certainly clue to
their greater conspicuousness having favoured their dispersal,
though it may also have arisen in part from the chemical
changes of chlorophyll during ripening and decay producing
red tints as in many fading leaves. Yet the comparative
scarcity of yellow in fruits, while it is the most common tint
of fading leaves, is against this supposition.

There are, however, a few instances of coloured fruits which
do not seem to be intended to be eaten ; such are the colo-
cynth plant (Cucumis colocynthus), which has a beautiful fruit
the size and colour of an orange, but nauseous beyond descrip-
tion to the taste. It has a hard rind, and may perhaps be dis-
persed by being blown along the ground, the colour being an
adventitious product ; but it is quite possible, notwithstanding
its repulsiveness to us, that it may be eaten by some animals.
With regard to the fruit of another plant, Calotropis
procera, there is less doubt, as it is dry and full of thin,
flat-winged seeds, with fine silky filaments, eminently adapted
for wind -dispersal ; yet it is of a bright yellow colour, as
large as an apple, and therefore very conspicuous. Here,
therefore, we seem to have colour which is a mere by-
product of the organism and of no use to it ; but such
cases are exceedingly rare, and this rarity, when compared
with the great abundance of cases in which there is an
obvious purpose in the colour, adds weight to the evidence
in favour of the theory of the attractive coloration of edible
fruits in order that birds and other animals may assist in
their dispersal. Both the above-named plants are natives of
Palestine and the adjacent arid countries.1

The Colours of Flowers.
Flowers are much more varied in their colours than fruits,
1 Canon Tristram's Natural History of the Bible, pp. 483, 484.

xi THE SPECIAL COLOURS OF PLANTS 309

as they are more complex and more varied in form and
structure ; yet there is some parallelism between them in both
respects. Flowers are frequently adapted to attract insects
as fruits are to attract birds, the object being in the former to
secure cross -fertilisation, in the latter dispersal; while just
as colour is an index of the edibility of fruits which supply
pulp or juice to birds, so are the colours of flowers an indica-
tion of the presence of nectar or of pollen which are devoured
by insects.

The main facts and many of the details, as to the relation
of insects to flowers, were discovered by Sprengel in 1793.
He noticed the curious adaptation of the structure of many
flowers to the particular insects which visit them ; he proved
that insects do cross-fertilise flowers, and he believed that this
was the object of the adaptations, while the presence of nectar
and pollen ensured the continuance of their visits ; yet he
missed discovering the use of this cross-fertilisation. Several
writers at a later period obtained evidence that cross-fertilisa-
tion of plants was a benefit to them ; but the wide generality
of this fact and its intimate connection with the numerous
and curious adaptations discovered by Sprengel, was first
shown by Mr. Darwin, and has since been demonstrated by a
vast mass of observations, foremost among which are his own
researches on orchids, primulas, and other plants.1

By an elaborate series of experiments carried on for many
years Mr. Darwin demonstrated the great value of cross-
fertilisation in increasing the rapidity of growth, the strength
and vigour of the plant, and in adding to its fertility. This
effect is produced immediately, not as he expected would be
the case, after several generations of crosses. He planted seeds
from cross-fertilised and self-fertilised plants on two sides of
the same pot exposed to exactly similar conditions, and in
most cases the difference in size and vigour was amazing,
while the plants from cross -fertilised parents also produced
more and finer seeds. These experiments entirely confirmed
the experience of breeders of animals already referred to
(p. 160), and led him to enunciate his famous aphorism,

1 For a complete historical account of this subject with full references to
all the works upon it, see the Introduction to Hermann Miiller's Fertilisation
of Flowers, translated by D'Arcy W. Thompson.

310 DARWINISM

"Nature abhors perpetual self -fertilisation.1 In this principle
we appear to have a sufficient reason for the various con-
trivances by which so many flowers secure cross-fertilisation,
either constantly or occasionally. These contrivances are so
numerous, so varied, and often so highly complex and extra-
ordinary, that they have formed the subject of many elaborate
treatises, and have also been amply popularised in lectures
and handbooks. It will be unnecessary, therefore, to give
details here, but the main facts will be summarised in order
to call attention to some difficulties of the theory which seem
to require further elucidation.

Modes of securing Cross- Fertilisation.

When we examine the various modes in which the cross-
fertilisation of flowers is brought about, we find that some are
comparatively simple in their operation and needful adjust-
ments, others highly complex. The simple methods belong to
four principal classes : — (1) By dichogamy — that is, by the
anthers and the stigma becoming mature or in a fit state for
fertilisation at slightly different times on the same plant. The
result of this is that, as plants in different stations, on different
soils, or exposed to different aspects flower earlier or later, the
mature pollen of one plant can only fertilise some jilant
exposed to somewhat different conditions or of different con-
stitution, whose stigma Avill be mature at the same time ; and
this difference has been shown by Darwin to be that which is
adapted to secure the fullest benefit of cross -fertilisation.
This occurs in Geranium pratense, Thymus serpyllum, Arum
maculatum, and many others. (2) By the floAver being
self-sterile with its own pollen, as in the crimson flax. This
absolutely prevents self-fertilisation. (3) By the stamens and
anthers being so placed that the pollen cannot fall upon the
stigma, while it does fall upon a visiting insect which carries
it to the stigma of another flower. This effect is produced in
a variety of very simple ways, and is often aided by the
motion of the stamens which bend down out of the way of
the stigmas before the pollen is ripe, as in Malva sylvestris
(see Fig. 28). (4) By the male and female flowers being on

1 For the full detail of liis experiments, see Cross- and Self -Fertilisation
of Plants, 1876.

THE SPECIAL COLOURS OF PLANTS

311

different plants, forming the class Dioecia of Linnaeus. In these
cases the pollen may he carried to the stigmas either by the
wind or by the agency of insects.

Now these four methods are all apparently very simple,
and easily produced by varia-
tion and selection. They are
applicable to flowers of any
shape, requiring only such size
and colour as to attract insects,
and some secretion of nectar
to ensure their repeated visits,
characters common to the great
majority of flowers. All these
methods are common, except
perhaps the second ; but there
are many flowers in which the Fig. 28.

pollen irom another plant IS adapted for insect- adapted for self- '
prepotent Over the pollen from fertilisation. fertilisation.

the same flower, and this has nearly the same effect as self-
sterility if the flowers are frequently crossed by insects. We
cannot help asking, therefore, why have other and much
more elaborate methods been needed? And how have the
more complex arrangements of so many flowers been brought
about ? Before attempting to answer these questions, and in
order that the reader may appreciate the difficulty of the
problem and the nature of the facts to be explained, it will be
necessary to give a summary of the more elaborate modes of
securing cross-fertilisation.

(1) We first have dimorphism and heteromorphism, the
phenomena of which have been already sketched in our
seventh chapter.

Here we have both a mechanical and a physiological
modification, the stamens and pistil being variously modified
in length and position, while the different stamens in the same
flower have widely different degrees of fertility when applied
to the same stigma, — a phenomenon which, if it were not so
well established, would have appeared in the highest degree
improbable. The most remarkable case is that of the three
different forms of the loosestrife (Lythrum salicaria) here
figured (Fig. 29 on next page).

312

DARWINISM

(2) Some flowers have irritable stamens which, when their

bases are touched by an insect, spring up and dust it with
pollen. This occurs in our common berberry.

xi THE SPECIAL COLOURS OF PLANTS 313

(3) In others there are levers or processes by which the
anthers are mechanically brought clown on to the head or
back of an insect entering the flower, in such a position as to
be carried to the stigma of the next flower it visits. This
may be well seen in many species of Salvia and Erica.

(4) In some there is a sticky secretion which, getting on
to the proboscis of an insect, carries away the pollen, and
applies it to the stigma of another flower. This occurs in our
common milkwort (Polygala vulgaris).

(5) In papilionaceous plants there are many complex ad-
justments, such as the squeezing out of pollen from a
receptacle on to an insect, as in Lotus corniculatus, or the
sudden springing out and exploding of the anthers so as
thoroughly to dust the insect, as in Medicago falcata, this
occurring after the stigma has touched the insect and taken
off some pollen from the last flower.

(6) Some flowers or spathes form closed boxes in which
insects find themselves entrapped, and when they have fertilised
the flower, the fringe of hairs opens and allows them to escape.
This occurs in many species of Arum and Aristolochia.

(7) Still more remarkable are the traps in the flower of
Asclepias which catch flies, butterflies, and wasps by the legs,
and the wonderfully complex arrangements of the orchids.
One of these, our common Orchis pyramidalis, may be briefly
described to show how varied and beautiful are the arrange-
ments to secure cross-fertilisation. The broad trifid lip of
the flower offers a support to the moth which is attracted
by its sweet odour, and two ridges at the base guide the
proboscis with certainty to the narrow entrance of the
nectary. When the proboscis has reached the end of the
spur, its basal portion depresses the little hinged rostellum
that covers the saddle -shaped sticky glands to which the
pollen masses (pollinia) are attached. On the proboscis
being withdrawn, the two pollinia stand erect and parallel,
firmly attached to the proboscis. In this position, however,
they would be useless, as they would miss the stigmatic
surface of the next flower visited by the moth. But as
soon as the proboscis is ' withdrawn, the two pollen masses
begin to diverge till they are exactly as far apart as are the
stigmas of the flower ; and then commences a second move-

314

DARWINISM

Fig. 30.— Orchis pyramidalis.
Description of Figure.
a ... anther. I r . . rostellum I V . guiding ridges on labellum.

s,s ... stigma. | I . . labellum or lip. | n . nectary.

A. Front view, with all the sepals and petals removed, except the labellum.

B. Side view, with all the sepals and petals removed and the upper part of the flower

C. The two pollinia attached to the saddle-shaped viscid disc. [bisected.

D. The disc after the first act of contraction.

E. The disc seen from above with one pollinium removed.

F. The pollinia removed by the insertion of a needle into the nectary.

G. The same pollinia after depression has taken place.

xi THE SPECIAL COLOURS OF PLANTS 315

ment which brings them down till they project straight for-
ward nearly at right angles to their first position, so as exactly
to hit against the stigmatic surfaces of the next flower visited
on which they leave a portion of their pollen. The whole of
these motions take about half a minute, and in that time the
moth will usually have flown to another plant, and thus effect
the most beneficial kind of cross-fertilisation.1 This descrip-
tion will be better understood by referring to the illustration
opposite, from Darwin's Fertilisation of Orchids (Fig. 30).

The Interpretation of these Facts.

Having thus briefly indicated the general character of the
more complex adaptations for cross-fertilisation, the details of
which are to be found in any of the numerous works on the
subject,2 we find ourselves confronted with the very puzzling
questiou— Why were these innumerable highly complex
adaptations produced, when the very same result may be
effected — and often is effected — by extremely simple means 1
Supposing, as we must do, that all flowers were once of
simple and regular forms, like a buttercup or a rose, how
did such irregular and often complicated flowers as the
papilionaceous or pea family, the labiates or sage family, and
the infinitely varied and fantastic orchids ever come into ex-
istence 1 No cause has yet been suggested but the need of
attracting insects to cross-fertilise them; yet the attractive-
ness of regular flowers with bright colours and an ample
supply of nectar is equally great, and cross-fertilisation can be
quite as effectively secured in these by any of the four simple
methods already described. Before attempting to suggest a
possible solution of this difficult problem, we have yet to pass
in review a large body of curious adaptations connected with
insect fertilisation, and will first call attention to that portion
of the phenomena which throw some light upon the special
colours of flowers in their relation to the various kinds of
insects which visit them. For these facts we are largely in-

1 See Darwin's Fertilisation of Orchids for the many extraordinary and
complex arrangements in these plants.

2 The English reader may consult Sir John Lubbock's British Wild
Flowers in Relation to Insects, and H. Midler's great and original work, The
Fertilisation of Floivers.

316 DARWINISM

debted to the exact and long-continued researches of Professor
Hermann Midler.

Summary of Additional Facts bearing on Insect Fertilisation.

1. That the size and colour of a flower are important
factors in determining the visits of insects, is shown by the
general fact of more insects visiting conspicuous than incon-
spicuous flowers. As a single instance, the handsome Geranium
palustre was observed by Professor Midler to be visited by
sixteen different species of insects, the equally showy G.
pratense by thirteen species, while the smaller and much
less conspicuous G. molle was visited by eight species, and
G-. pusillum by only one. In many cases, however, a flower
may be very attractive to only a few species of insects ; and
Professor Midler states, as the result of many years' assiduous
observation, that "a species of flower is the more visited by
insects the more conspicuous it is."

2. Sweet odour is usually supplementary to the attraction of
colour. Thus it is rarely present in the largest and most gaudily
coloured flowers which inhabit open places, such as poppies,
pseonies, sunflowers, and many others; while it is often the
accompaniment of inconspicuous flowers, as the mignonette ; of
such as grow in shady places, as the violet and primrose ; and
especially of white or yellowish flowers, as the white jasmine,
clematis, stephanotis, etc.

3. White floAvers are often fertilised by moths, and very
frequently give out their scent only by night, as in our butterfly-
orchis (Habenaria chlorantha) ; and they sometimes open only at
night, as do many of the evening primroses and other flowers.
These flowers are often long tubed in accordance with the
length of the moths' probosces, as in the genus Pancratium,
our butterfly orchis, white jasmine, and a host of others.

4. Bright red flowers are very attractive to butterflies, and
are sometimes specially adapted to be fertilised by them, as
in many pinks (Dianthus deltoides, D. superbus, D. atrorubens),
the corn-cockle (Lychnis Githago), and many others. Blue
flowers are especially attractive to bees and other hymenoptera
(though they frequent flowers of all colours), no less than sixty-
seven species of this order having been observed to visit the
common "sheep's-bit" (Jasione montana). Dull yellow or

xi THE SPECIAL COLOURS OF PLANTS 317

brownish flowers, some of which smell like carrion, are
attractive to flies, as the Arum and Aristolochia ; while the
dull purplish flowers of the Scrophularia are specially attrac-
tive to wasps.

5. Some flowers have neither scent nor nectar, and yet
attract insects by sham nectaries ! In the herb-paris (Paris
quadrifolia) the ovary glistens as if moist, and flies alight on it
and carry away pollen to another flower ; while in grass of
parnassus (Parnassia palustris) there are a number of small
stalked yellow balls near the base of the flower, which look
like drops of honey but are really dry. In this case there is
a little nectar lower down, but the special attraction is a
sham ; and as there are fresh broods of insects every year, it
takes time for them to learn by experience, and thus enough
are always deceived to effect cross -fertilisation.1 This is
analogous to the case of the young birds, which have to learn
by experience the insects that are inedible, as explained at
page 253.

6. Many flowers change their colour as soon as fertilised ;
and this is beneficial, as it enables bees to avoid wasting time
in visiting those blossoms which have been already fertilised
and their nectar exhausted. The common lungwort (Pul-
monaria officinalis), is at first red, but later turns blue ; and
H. Midler observed bees visiting many red flowers in
succession, but neglecting the blue. In South Brazil there
is a species of Lantana, whose flowers are yellow the first day,
orange the second, and purple the third ; and Dr. Fritz
Midler observed that many butterflies visited the yellow
flowers only, some both the yellow and the orange flowers,
but none the purple.

7. Many flowers have markings which serve as guides to
insects ; in some cases a bright central eye, as in the borage
and forget-me-not ; or lines or spots converging to the centre,
as in geraniums, pinks, and many others. This enables
insects to go quickly and directly to the opening of the
flower, and is equally important in aiding them to obtain a
better supply of food, and to fertilise a larger number of
flowers.

8. Flowers have been specially adapted to the kinds of

1 Miiller's Fertilisation of Floivers, p. 248.

318 DARWINISM

insects that most abound where they grow. Thus the gentians
of the lowlands are adapted to bees, those of the high alps to
butterflies only ; and while most species of Khinanthus (a
genus to which our common " yellow rattle " belongs) are bee-
flowers, one high alpine species (R. alpinus) has been also
adapted for fertilisation by butterflies only. The reason of
this is, that in the high alps butterflies are immensely more
plentiful than bees, and flowers adapted to be fertilised by
bees can often have their nectar extracted by butterflies
without effecting cross -fertilisation. It is, therefore, im-
portant to have a modification of structure which shall make
butterflies the fertilisers, and this in many cases has been done.1

9. Economy of time is very important both to the insects
and the flowers, because the fine working days are com-
paratively few, and if no time is wasted the bees will get
more honey, and in doing so will fertilise more flowers. Now,
it has been ascertained by several observers that many insects,
bees especially, keep to one kind of flower at a time, visiting
hundreds of blossoms in succession, and passing over other
species that may be mixed with them. They thus acquire
quickness in going at once to the nectar, and the change of
colour in the flower, or incipient withering when fertilised,
enables them to avoid those flowers that have already had
their honey exhausted. It is probably to assist the insects in
keeping to one flower at a time, which is of vital importance
to the perpetuation of the species, that the flowers which
bloom intermingled at the same season are usually very dis-
tinct both in form and colour. In the sandy districts of
Surrey, in the early spring, the copses are gay with three
flowers — the primrose, the wood -anemone, and the lesser
celandine, forming a beautiful contrast, lvhile at the same
time the purple and the white dead-nettles abound on hedge
banks. A little later, in the same copses, we have the blue
wild hyacinth (Scilla nutans), the red campion (Lychnis
dioica), the pure white great starwort (Stellaria Holosteum),
and the yellow dead-nettle (Lamium Galeobdolon), all distinct
and well-contrasted flowers. In damp meadows in summer
we have the ragged robin (Lychnis Floscuculi), the spotted
orchis (0. maculata), and the yellow rattle (Ehinanthus

1 " Alpeublumen," by D. H. Miiller. See Nature, vol. xxiii. p. 333.

xi THE SPECIAL COLOURS OF PLANTS 319

Crista -galli) ; while in drier meadows we have coAvslips,
ox-eye daisies, and buttercups, all very distinct both in form
and colour. So in cornfields we have the scarlet poppies, the
purple corn-cockle, the yellow corn -mary gold, and the blue
cornflower; while on our moors the purple heath and the
dwarf gorse make a gorgeous contrast. Thus the difference
of colour which enables the insect to visit with rapidity and
unerring aim a number of flowers of the same kind in suc-
cession, serves to adorn our meadows, banks, woods, and
heaths with a charming variety of floral colour and form at
each season of the year.1

Fertilisation of Flowers by Birds.

In the temperate regions of the Northern Hemisphere,
insects are the chief agents in cross-fertilisation when this is
not effected by the wind ; but in warmer regions, and in the
Southern hemisphere, birds are found to take a considerable
part in the operation, and have in many cases led to modifi-
cations in the form and colour of flowers. Each part of the
globe has special groups of birds which are flower-haunters.
America has the humming-birds (Trochilidse), and the smaller
group of the sugar -birds (Cserebidse). In the Eastern tropics
the sun-birds (Nectarineidse) take the place of the humming-
birds, and another small group, the flower-peckers (Dicseidse),
assist them. In the Australian region there are also two
flower-feeding groups, the Meliphagidse, or honey -suckers,
and the brush -tongued lories (Trichoglossidse). Recent re-
searches by American naturalists have shown that many
flowers are fertilised by humming-birds, such as passion-
flowers, trumpet -flowers, fuchsias, and lobelias ; while some,
as the Salvia splendens of Mexico, are specially adapted to
their visits. We may thus perhaps explain the number of
very large tubular flowers in the tropics, such as the huge
brugmansias and bignonias ; while in the Andes and in

1 This peculiarity of local distribution of colour in flowers may be com-
pared, as regards its purpose, with the recognition colours of animals. Just
as these latter colours enable the sexes to recognise each other, and thus avoid
sterile unions of distinct species, so the distinctive form and colour of each
species of flower, as compared with those that usually grow around it, enables
the fertilising insects to avoid carrying the pollen of one flower to the stigma
of a distinct species.

320

DARWINISM

Chile, where humming-birds are especially plentiful, we find
great numbers of red tubular flowers, often of large size and
apparently adapted to these little creatures. Such are the
beautiful Lapageria and Philesia, the grand Pitcairneas, and
the genera Fuchsia; Mitraria, Embothrium, Escallonia, Desfon-
tainea, Eccremocarpus, and many Gesneracese. Among the
most extraordinary modifications of flower structure adapted

Fig. 31.— Humming-bird fertilising Marcgravia nepenthoides.

to bird fertilisation are the species of Marcgravia, in which the
pedicels and bracts of the terminal portion of a pendent bunch
of flowers have been modified into pitchers which secrete
nectar and attract insects, while birds feeding on the nectar,
or insects, have the pollen of the overhanging flowers dusted
on their backs, and, carrying it to other flowers, thus cross-
fertilise them (see Illustration).

In Australia and New Zealand the fine "glory peas"
(Clianthus), the Sophora, Loranthus, many Epacriclere and
Myrtacese, and the large flowers of the New Zealand flax

xi THE SPECIAL COLOURS OF PLANTS 321

(Phormium tenax), are cross-fertilised by birds ; while in Natal
the fine trumpet -creeper (Tecoma capensis) is fertilised by
Nectarineas.

The great extent to which insect and bird agency is
necessary to flowers is well shown by the case of New
Zealand. The entire country is comparatively poor in species
of insects, especially in bees and butterflies which are the
chief flower fertilisers ; yet according to the researches of
local botanists no less than one -fourth of all the flowering-
plants are incapable of self-fertilisation, and, therefore, wholly
dependent on insect or bird agency for the continuance of
the species.

The facts as to the cross-fertilisation of flowers which have
now been very briefly summarised, taken in connection with
Darwin's, experiments proving the increased vigour and fer-
tility given by cross -fertilisation, seem amply to justify his
aphorism that " Nature abhors self -fertilisation," and his more
precise statement, that, "No plant is perpetually self -fertil-
ised ; " and this view has been upheld by Hildebrand, Delpino,
and other botanists.1

Self- Fertilisation of Flowers.

But all this time we have been only looking at one side of
the question, for there exists an abundance of facts which
seem to imply, just as surely, the utter uselessness of cross-
fertilisation. Let us, then, see what these facts are before pro-
ceeding further.

1. An immense variety of plants are habitually self -fer-
tilised, and their numbers probably far exceed those which
are habitually cross-fertilised by insects. Almost all the very
small or obscure flowered plants with hermaphrodite flowers
are of this kind. Most of these, however, may be insect
fertilised occasionally, and may, therefore, come under the rule
that no species are perpetually self-fertilised.

2. There are many plants, however, in which special
arrangements exist to secure self-fertilisation. Sometimes the
corolla closes and brings the anthers and stigma into contact ;
in others the anthers cluster round the stigmas, both maturing
together, as in many buttercups, stitchwort (Stellaria media),

1 See H. Muller's Fertilisation of Flowers, p. 18.
Y

322 DARWINISM

sandwort (Spergula), and some willow-herbs (Epilobium) ; or
they arch over the pistil, as in Galium aparine and Alisma
Plantago. The style is also modified to bring it into contact
with the anthers, as in the dandelion, groundsel, and many
other plants.1 All these, however, may be occasionally cross-
fertilised.

3. In other cases precautions are taken to prevent cross-
fertilisation, as in the numerous cleistogamous or closed flowers.
These occur in no less than fifty -five different genera, belonging
to twenty -four natural orders, and in thirty-two of these genera
the normal flowers are irregular, and have therefore been
specially modified for insect fertilisation.2 These flowers appear
to be degradations of the normal flowers, and are closed up by
various modifications of the petals or other parts, so that it is
impossible for insects to reach the interior, yet they produce
seed in abundance, and are often the chief means by which
the species is continued. Thus, in our common dog-violet the
perfect flowers rarely produce seed, while the rudimentary
cleistogamic flowers do so in abundance. The sweet violet also
produces abundance of seed from its cleistogamic flowers, and
few from its perfect flowers ; but in Liguria it produces only
perfect flowers which seed abundantly. No case appears to
be known of a plant which has cleistogamic flowers only, but
a small rush (Juncus bufonius) is in this condition in some
parts of Russia, while in other parts perfect flowers are also
produced.3 Our common henbit dead-nettle (Lamium amplex-
icaule) produces cleistogamic flowers, as do also some orchids.
The advantage gained by the plant is great economy of
specialised material, since with very small flowers and very
little expenditure of pollen an abundance of seed is produced.

4. A considerable number of plants which have evidently
been specially modified for insect fertilisation have, by further

1 The above examples are taken from Rev. G. Henslow's paper on "Self-
Fertilisation of Plants," in Trans. Linn. Soc. Second series, Botany, vol. i.
pp. 317-398, with plate. Mr. H. 0. Forbes has shown that the same thing-
occurs among tropical orchids, in his paper " On the Contrivances for insuring
Self- Fertilisation in some Tropical Orchids," Journ. Linn. Soc, xxi. p. 538.

2 These are the numbers given by Darwin, but I am informed by Mr.
Hemsley that many additions have been since made to the list, and that
cleistogamic flowers probably occur in nearly all the natural orders.

3 For a full account of cleistogamic flowers, see Darwin's Forms of Flowers,
chap. viii.

xi THE SPECIAL COLOURS OF PLANTS 323

modification, become quite self-fertile. This is the case with
the garden-pea, and also with our beautiful bee-orchis, in which
the pollen-masses constantly fall on to the stigmas, and the
flower, being thus self-fertilised, produces abundance of capsules
and of seed. Yet in many of its close allies insect agency is
absolutely required ; but in one of these, the fly-orchis, com-
paratively very little seed is produced, and self-fertilisation
would therefore be advantageous to it. When garden-peas
were artificially cross-fertilised by Mr. Darwin, it seemed to do
them no good, as the seeds from these crosses produced less
vigorous plants than seed from those which were self-fertilised ;
a fact directly opposed to what usually occurs in cross-fer-
tilised plants.

5. As opposed to the theory that there is any absolute need
for cross-fertilisation, it has been urged by Mr. Henslow and
others that many self-fertilised plants are exceptionally vigorous,
such as groundsel, chickweed, sow-thistle, buttercups, and other
common weeds ; while most plants of world-wide distribution
are self- fertilised, and these have proved themselves to be best
fitted to survive in the battle of life. More than fifty species
of common British plants are very widely distributed, and all
are habitually self -fertilised.1 That self -fertilisation has some
great advantage is shown by the fact that it is usually the
species which have the smallest and least conspicuous flowers
which have spread widely, while the large and showy flowered
species of the same genera or families, which require insects to
cross-fertilise them, have a much more limited distribution.

6. It is now believed by some botanists that many in-
conspicuous and imperfect flowers, including those that are
wind-fertilised, such as plantains, nettles, sedges, and grasses,
do not represent primitive or undeveloped forms, but are
degradations from more perfect flowers which were once
adapted to insect fertilisation. In almost every order we find
some plants which have become thus reduced or degraded for
wind or self-fertilisation, as Poterium and Sanguisorba among
the Eosacese ; Avhile this has certainly been the case in the
cleistogamic flowers. In most of the above-mentioned plants
there are distinct rudiments of petals or other floral organs,

1 Henslow's "Self-Fertilisation," Trans. Linn. Soc. Second series, Botany,
vol. i. p. 391.

324 DARWINISM

and as the chief use of these is to attract insects, they could
hardly have existed in primitive flowers.1 We know, moreover,
that when the petals cease to be required for the attraction of

1 The Eev. George Henslow, in his Origin of Floral Structures, says :
" There is little doubt but that all wind-fertilised angiosperms are degradations
from insect - fertilised flowers. . . . Poterium sanguisorba is anemophilous ;
and Sanguisorba officinalis presumably was so formerly, but has reacquired
an entomophilous habit ; the whole tribe Poterieee being, in fact, a degraded
group which has descended from Potentillese. Plantains retain their corolla
but in a degraded form. Junceaa are degraded Lilies ; while Cyperaceae and
Grammes among monocotyledons may be ranked with Amentiferae among
dicotyledons, as representing orders which have retrograded very far from
the entomophilous forms from which they were possibly and probably de-
scended" (p. 266).

"The genus Plantago, like Thalictrum mimes, Poterium, and others, well
illustrate the change from an entomophilous to the anemophilous state.
P. lanceolata has polymorphic flowers, and is visited by pollen-seeking insects,
so that it can be fertilised either by insects or the wind. P. media illustrates
transitions in point of structure, as the filaments are pink, the anthers
motionless, and the pollen grains aggregated, and it is regularly visited by
Bombus terrestris. On the other hand, the slender filaments, versatile anthers,
powdery pollen, and elongated protogynous style are features of other species
indicating anemophily ; while the presence of a degraded corolla shows its
ancestors to have been entomophilous. P. media, therefore, illustrates, not
a primitive entomophilous condition, but a return to it ; just as is the case
with Sanguisorba officinalis and Salix Caprea ; but these show no capacity of
restoring the corolla, the attractive features having to be borne by the calyx,
which is purplish in Sanguisorba, by the pink filaments of Plantago, and by
the yellow anthers in the Sallow willow" (p. 271).

" The interpretation, then, I would offer of inconspicuousness and all kinds
of degradations is the exact opposite to that of conspicuousness and great
differentiations ; namely, that species with minute flowers, rarely or never
visited by insects, and habitually self-fertilised, have primarily arisen through
the neglect of insects, and have in consequence assumed their present floral
structures " (p. 282).

In a letter just received from Mr. Henslow, he gives a few additional
illustrations of his views, of which the following are the most important :
" Passing to Incompletae, the orders known collectively as ' Cyclospermea? '
are related to Caryophyllere ; and to my mind are degradations from it, of
which Orache is anemophilous. Cupulii'eraa have an inferior ovary and rudi-
mentary calyx-limb on the top. These, as far as I know, cannot be inter-
preted except as degradations. The whole of Monocotyledons appear to me
(from anatomical reasons especially) to be degradations from Dicotyledons,
and primarily through the agency of growth in water. Many subsequently
became terrestrial, but retained the effects of their primitive habitat through
heredity. The 3-merous perianth of grasses, the parts of the flower being in
whorls, point to a degradation from a sub -liliaceous condition."

Mr. Henslow informs me that he has long held these views, but, as far as
he knows, alone. Mr. Grant Allen, however, set forth a similar theory in his
Vignettes from Nature (p. 15) and more fully in The Colours of Floiuers
(chap, v.), where he develops it fully and uses similar arguments to those of
Mr. Henslow.

xi THE SPECIAL COLOURS OF PLANTS 325

insects, they rapidly diminish in size, lose their bright colour
or almost wholly disappear 1

Difficulties and Contradictions.

The very bare summary that has now been given of the
main facts relating to the fertilisation of flowers, will have
served to show the vast extent and complexity of the inquiry,
and the extraordinary contradictions and difficulties which it
presents. We have direct proof of the beneficial results of
intercrossing in a great number of cases ; we have an over-
whelming mass of facts as to the varied and complex structure
of flowers evidently adapted to secure this intercrossing by
insect agency ; yet we see many of the most vigorous plants
which spread widely over the globe, with none of these
adaptations, and evidently depending on self-fertilisation for
their continued existence and success in the battle of life.
Yet more extraordinary is it to find numerous cases in which
the special arrangements for cross-fertilisation appear to have
been a failure, since they have either been supplemented by
special means for self-fertilisation, or have reverted back in
various degrees to simpler forms in which self-fertilisation
becomes the rule. There is also a further difficulty in the
highly complex modes by which cross-fertilisation is often
brought about ; for we have seen that there are several very
effective yet very simple modes of securing intercrossing,
involving a minimum of change in the form and structure of
the flower; and when we consider that the result attained
with so much cost of structural modification is by no means
an unmixed good, and is far less certain in securing the per-
petuation of the species than is self-fertilisation, it is most
puzzling to find such complex methods resorted to, some-
times to the extent of special precautions against the possi-
bility of self-fertilisation ever taking place. Let us now see
whether any light can be thrown on these various anomalies
and contradictions.

Intercrossing not necessarily Advantageous.

No one was more fully impressed than Mr. Darwin with
the beneficial effects of intercrossing on the vigour and fertility

1 H. Miiller gives ample proof of this in his Fertilisation of Flowers.

326 DARWINISM

of the species or race, yet he clearly saw that it was not
always and necessarily advantageous. He says : " The most
important conclusion at which I have arrived is, that the mere
act of intercrossing by itself does no good. The good
depends on the individuals which are crossed differing slightly
in constitution, owing to their progenitors having been sub-
jected during several generations to slightly different con-
ditions. This conclusion, as Ave shall hereafter see, is closely
connected with various important physiological problems, such
as the benefit derived from slight changes in the conditions of
life."1 Mr. Darwin has also adduced much direct evidence
proving that slight changes in the conditions of life are
beneficial to both animals and plants, maintaining or restoring
their vigour and fertility in the same way as a favourable
cross seems to restore it.2 It is, I believe, by a careful
consideration of these two classes of facts that we shall find
the clue to the labyrinth in which this subject has appeared
to involve us.

Supposed Evil Results of Close Interbreeding.

Just as we have seen that intercrossing is not necessarily
good, we shall be forced to admit that close interbreeding is
not necessarily bad. Our finest breeds of domestic animals
have been thus produced, and by a careful statistical inquiry
Mr. George Darwin has shoAvn that the most constant and
long- continued intermarriages among the British aristocracy
have produced no prejudicial results. The rabbits on Porto
Santo are all the produce of a single female ; they have lived
on the same small island for 470 years, and they still abound
there and appear to be vigorous and healthy (see p. 161).

We have, however, on the other hand, overwhelming
evidence that in many cases, among our domestic animals and
cultivated plants, close interbreeding does produce bad results,
and the apparent contradiction may perhaps be explained on
the same general principles, and under similar limitations, as
were found to be necessary in defining the value of inter-
crossing. It appears probable, then, that it is not inter-
breeding in itself that is hurtful, but interbreeding without

1 Cross- and Self -Fertilisation, p. 27.

2 Animals and Plants, vol. ii. p. 145.

XI THE SPECIAL COLOURS OF PLANTS 327

rigid selection or some change of conditions. Under nature,
as in the case of the Porto Santo rabbits, the rapid increase of
these animals would in a very few years stock the island with
a full population, and thereafter natural selection would act
powerfully in the preservation only of the healthiest and the
most fertile, and under these conditions no deterioration
would occur. Among the aristocracy there has been a
constant selection of beauty, which is generally synonymous
with health, while any constitutional infertility has led to the
extinction of the family. With domestic animals the selec-
tion practised is usually neither severe enough nor of the
right kind. There is no natural struggle for existence, but
certain points of form and colour characteristic of the breed
are considered essential, and thus the most vigorous or the
most fertile are not always those which are selected to
continue the stock. In nature, too, the species always extends
over a larger area and consists of much greater numbers, and
thus a difference of constitution soon arises in different parts
of the area, which is wanting in the limited numbers of pure
bred domestic animals. From a consideration of these varied
facts we conclude that an occasional disturbance of the organic
equilibrium is what is essential to keep up the vigour and
fertility of any organism, and that this disturbance may be
equally well produced either by a cross between individuals
of somewhat different constitutions, or by occasional slight
changes in the conditions of life. Now plants which have
great powers of dispersal enjoy a constant change of con-
ditions, and can, therefore, exist permanently, or at all events,
for very long periods, without intercrossing; while those
which have limited powers of dispersal, and are restricted to
a comparatively small and uniform area, need an occasional
cross to keep up their fertility and general vigour. "We
should, therefore, expect that those groups of plants which are
adapted both for cross- and self- fertilisation, which have showy
flowers and possess great powers of seed-dispersal, would be
the most abundant and most widely distributed ; and this we
find to be the case, the Compositse possessing all these charac-
teristics in the highest degree, and being the most generally
abundant group of plants with conspicuous flowers in all parts
of the world.

323 DARWINISM

Hoio the Struggle for Existence Acts among Flowers.

Let us now consider what will be the action of the struggle
for existence under the conditions we have seen to exist.

Everywhere and at all times some species of plants will be
dominant and aggressive ; while others will be diminishing in
numbers, reduced to occupy a smaller area, and generally
having a hard struggle to maintain themselves. Whenever
a self-fertilising plant is thus reduced in numbers it will be
in danger of extinction, because, being limited to a small
area, it will suffer from the effects of too uniform conditions
which will produce weakness and infertility. But while this
change is in progress, any crosses between individuals of
slightly different constitution will be beneficial, and all varia-
tions favouring either insect agency on the one hand, or
wind-dispersal of pollen on the other, will lead to the pro-
duction of a somewhat stronger and more fertile stock. In-
creased size or greater brilliancy of the flower, more abundant
nectar, sweeter odour, or adaptations for more effectual cross-
fertilisation would all be preserved, and thus would be initiated
some form of specialisation for insect agency in cross -fertil-
isation ; and in every different species so circumstanced the
result would be different, depending as it would on many
and complex combinations of variation of parts of the flower,
and of the insect species which most abounded in the district.

Species thus favourably modified might begin a new era
of development, and, while spreading over a somewhat wider
area, give rise to new varieties or species, all adapted in
various degrees and modes to secure cross -fertilisation by
insect agency. But in course of ages some change of condi-
tions might prove adverse. Either the insects required might
diminish in numbers or be attracted by other competing
flowers, or a change of climate might give the advantage
to other more vigorous plants. Then self-fertilisation with
greater means of dispersal might be more advantageous ; the
flowers might become smaller and more numerous ; the seeds
smaller and lighter so as to be more easily dispersed by the
wind, while some of the special adaptations for insect fertilis-
ation being useless would, by the absence of selection and by
the law of economy of growth, be reduced to a rudimentary

xi THE SPECIAL COLOURS OF PLANTS 329

form. With these modifications the species might extend its
range into new districts, thereby obtaining increased vigour
by the change of conditions, as appears to have been the case
with so many of the small flowered self-fertilised plants. Thus
it might continue to exist for a long series of ages, till under
other changes — geographical or biological — it might again
suffer from competition or from other adverse circumstances,
and be at length again confined to a limited area, or reduced
to very scanty numbers.

But when this cycle of change had taken place, the species
would be very different from the original form. The flower
would have been at one time modified to favour the visits
of insects and to secure cross -fertilisation by their aid, and
when the need for this passed away, some portions of these
structures would remain, though in a reduced or rudi-
mentary condition. But when insect agency became of
importance a second time, the new modifications would
start from a different or more advanced basis, and thus a
more complex result might be produced. Owing to the
unequal rates at which the reduction of the various parts
might occur, some amount of irregularity in the flower might
arise, and on a second development towards insect cross-
fertilisation this irregularity, if useful, might be increased by
variation and selection.

The rapidity and comparative certainty with which such
changes as are here supposed do really take place, are well
shown by the great differences in floral structure, as regards
the mode of fertilisation, in allied genera and species, and even
in some cases in varieties of the same species. Thus in the
Banunculacese we find the conspicuous part of the flower to be
the petals in Ranunculus, the sepals in Helleborus, Anemone,
etc., and the stamens in most species of Thalictrum. In all
these we have a simple regular flower, but in Aquilegia it is
made complex by the spurred petals, and in Delphinium and
Aconitum it becomes quite irregular. In the more simple class
self-fertilisation occurs freely, but it is prevented in the more
complex flowers by the stamens maturing before the pistil.
In the Caprifoliaceae we have small and regular greenish
flowers, as in the moschatel (Acloxa) ; more conspicuous regular
open flowers without honey, as in the elder (Sambucus) ; and

330 DARWINISM chap.

tubular flowers increasing in length and irregularity, till in
some, like our common honeysuckle, they are adapted for
fertilisation by moths only, with abundant honey and
delicious perfume to attract them. In the Scrophulariacea?
we find open, almost regular flowers, as Veronica and
Verbascum, fertilised by flies and bees, but also self-fertilised ;
Scrophularia adapted in form and colour to be fertilised by
wasps ; and the more complex and irregular flowers of
Linaria, Ehinanthus, Melampyrum, Peclicularis, etc., mostly
adapted to be fertilised by bees.

In the genera Geranium, Polygonum, Veronica, and several
others there is a gradation of forms from large and bright
to small and obscure coloured flowers, and in every case the
former are adapted for insect fertilisation, often exclusively,
while in the latter self-fertilisation constantly occurs. In the
yellow rattle (Rhinanthus Crista-galli) there are two forms
(which have been named major and minor), the larger and
more conspicuous adapted to insect fertilisation only, the
smaller capable of self-fertilisation ; and two similar forms exist
in the eyebright (Euphrasia officinalis). In both these cases
there are special modifications in the length and curvature
of the style as well as in the size and shape of the corolla ;
and the two forms are evidently becoming each adapted to
special conditions, since in some districts the one, in other
districts the other is most abundant.1

These examples show us that the kind of change suggested
above is actually going on, and has presumably always been
going on in nature throughout the long geological epochs
during which the development of flowers has been progressing.
The two great modes of gaining increased vigour and fertility
— intercrossing and dispersal over wider areas — have been
resorted to again and again, under the pressure of a constant
struggle for existence and the need for adaptation to ever-
changing conditions. During all the modifications that ensued,
useless parts were reduced or suppressed, owing to the absence
of selection and the principle of economy of growth ; and thus
at each fresh adaptation some rudiments of old structures were

1 Miiller's Fertilisation of Floivers, pp. 448, 455. Other cases of recent
degradation and readaptation to insect - fertilisation are given by Professor
Henslow (see footnote, p. 324).

xi THE SPECIAL COLOURS OP PLANTS 331

re -developed, but not unfrequently in a different form and for
a distinct purpose.

The chief types of flowering plants have existed during the
millions of ages of the whole tertiary period, and during this
enormous lapse of time many of them may have been modified
in the direction of insect fertilisation, and again into that of
self-fertilisation, not once or twice only, but perhaps scores or
even hundreds of times ; and at each such modification a
difference in the environment may have led to a distinct
line of development. At one epoch the highest specialisation
of structure in adaptation to a single species or group of insects
may have saved a plant from extinction; while, at other times,
the simplest mode of self-fertilisation, combined with greater
powers of dispersal and a constitution capable of supporting
diverse physical conditions, may have led to a similar result.
With 'some groups the tendency seems to have been almost
continuously to greater and greater specialisation, while with
others a tendency to simplification and degradation has resulted
in such plants as the grasses and sedges.

We are now enabled dimly to perceive how the curious
anomaly of very simple and very complex methods of securing
cross-fertilisation — both equally effective — may have been
brought about. The simple modes may be the result of a
comparatively direct modification from the more primitive
types of flowers, which were occasionally, and, as it were,
accidentally visited and fertilised by insects ; while the more
complex modes, existing for the most part in the highly irregular
flowers, may result from those cases in which adaptation to
insect-fertilisation, and partial or complete degradation to self-
fertilisation or to wind -fertilisation, have again and again
recurred, each time producing some additional complexity,
arising from the working up of old rudiments for new pur-
poses, till there have been reached the marvellous flower
structures of the papilionaceous tribes, of the asclepiads, or of
the orchids.

We thus see that the existing diversity of colour and of
structure in flowers is probably the ultimate result of the
ever-recurring struggle for existence, combined with the ever-
changing relations between the vegetable and animal kingdoms
during countless ages. The constant variability of every part

332 DARWINISM

and organ, with the enormous powers of increase possessed by
plants, have enabled them to become again and again readjusted
to each change of condition as it occurred, resulting in that
endless variety, that marvellous complexity, and that ex-
quisite colouring which excite our admiration in the realm of
flowers, and constitute them the perennial charm and crowning
glory of nature.

Flowers the Product of Insect Agency.

In his Origin of Species, Mr. Darwin first stated that
flowers had been rendered conspicuous and beautiful in order
to attract insects, adding : " Hence we may conclude that, if
insects had not been developed on the earth, our plants would
not have been decked with beautiful flowers, but would have
produced only such poor flowers as we see on our fir, oak, nut,
and ash trees, on grasses, docks, and nettles, which are all
fertilised through the agency of the wind." The argument in
favour of this view is now much stronger than when he wrote ;
for not only have we reason to believe that most of these
Avind-fertilised flowers are degraded forms of flowers which
have once been insect fertilised, but we have abundant evidence
that whenever insect agency becomes comparatively ineffective,
the colours of the flowers become less bright, their size and
beauty diminish, till they are reduced to such small, greenish,
inconspicuous flowers as those of the rupture-wort (Herniaria
glabra), the knotgrass (Polygonum aviculare), or the cleisto-
gamic flowers of the violet. There is good reason to believe,
therefore, not only that flowers have been developed in order
to attract insects to aid in their fertilisation, but that, having
been once produced, in however great profusion, if the insect
races were all to become extinct, flowers (in the temperate
zones at all events) would soon dwindle away, and that
ultimately all floral beauty would vanish from the earth.

We cannot, therefore, deny the vast change which insects
have produced upon the earth's surface, and which has been
thus forcibly and beautifully delineated by Mr. Grant Allen :
" While man has only tilled a few level plains, a few great river
valleys, a few peninsular mountain slopes, leaving the vast mass
of earth untouched by his hand, the insect has spread himself
over every land in a thousand shapes, and has made the whole

xi THE SPECIAL COLOURS OF PLANTS 333

flowering creation subservient to his daily wants. His butter-
cup, his dandelion, and his meadow-sweet grow thick in every
English field. His thyme clothes the hillside ; his heather
purples the bleak gray moorland. High up among the alpine
heights his gentian spreads its lakes of blue ; amid the snows
of the Himalayas his rhododendrons gleam with crimson light.
Even the wayside pond yields him the white crowfoot and the
arrowhead, while the broad expanses of Brazilian streams are
beautified by his gorgeous water-lilies. The insect has thus
turned the whole surface of the earth into a boundless flower-
garden, which supplies him from year to year with pollen or
honey, and itself in turn gains perpetuation by the baits that
it offers for his allurement."1

Concluding Remarks on Colour in Nature.

In the last four chapters I have endeavoured to give a
general and systematic, though necessarily condensed view of
the part which is played by colour in the organic world. We
have seen in what infinitely varied ways the need of conceal-
ment has led to the modification of animal colours, whether
among polar snows or sandy deserts, -in tropical forests or in
the abysses of the ocean. We next find these general adapta-
tions giving way to more specialised types of coloration,
by which each species has become more and more harmonised
with its immediate surroundings, till we reach the most
curiously minute resemblances to natural objects in the leaf
and stick insects, and those which are so like flowers or moss
or birds' droppings that they deceive the acutest eye. We
have learnt, further, that these varied forms of protective
colouring are far more numerous than has been usually sus-
pected, because, what appear to be very conspicuous colours
or markings when the species is observed in a museum or in
a menagerie, are often highly protective when the creature is
seen under the natural conditions of its existence. From
these varied classes of facts it seems not improbable that
fully one-half of the species in the animal kingdom possess
colours which have been more or less adapted to secure for
them concealment or protection.

Passing onward we find the explanation of a distinct type
1 The Colour Sense, by Grant Allen, p. 95.

334 DARWINISM

of colour or marking, often superimposed upon protective
tints, in the importance of easy recognition by many animals
of their fellows, their parents, or their mates. By this need
we have been able to account for markings that seem calcu-
lated to make the animal conspicuous, when the general tints
and well-known habits of the whole group demonstrate the
need of concealment. Thus also we are able to explain the
constant symmetry in the markings of wild animals, as well as
the numerous cases in which the conspicuous colours are con-
cealed when at rest and only become visible during rapid motion.

In striking contrast to ordinary protective coloration we
have " warning colours," usually very conspicuous and often
brilliant or gaudy, which serve to indicate that their possess-
ors are either dangerous or uneatable to the usual enemies
of their tribe. This kind of coloration is probably more
prevalent than has been hitherto supposed, because in the
case of many tropical animals we are quite unacquainted with
their special and most dangerous enemies, and are also un-
able to determine whether they are or are not distasteful to
those enemies. As a kind of corollary to the "warning
colours," we find the extraordinary phenomena of "mimicry,"
in which defenceless species obtain protection by being mis-
taken for those which, from any cause, possess immunity from
attack. Although a large number of instances of warning
colour and of mimicry are now recorded, it is probably still
an almost unworked field of research, more especially in
tropical regions and among the inhabitants of the ocean.

The phenomena of sexual diversities of coloration next
engaged our attention, and the reasons why Mr. Darwin's theory
of " sexual selection," as regards colour and ornament, could
not be accepted were stated at some length, together with
the theory of animal coloration and ornament we propose
to substitute for it. This theory is held to be in harmony
with the general facts of animal coloration, while it entirely
dispenses with the very hypothetical and inadequate agency
of female choice in producing the detailed colours, patterns,
and ornaments, which in so many cases distinguish the male
sex.

If my arguments on this point are sound, they will dispose
also of Mr. Grant Allen's view of the direct action of the

xi THE SPECIAL COLOURS OF PLANTS 335

colour sense on the animal integuments.1 He argues that the
colours of insects and birds reproduce generally the colours of
the flowers they frequent or the fruits they eat, and he
adduces numerous cases in which flower-haunting insects and
fruit- eating birds are gaily coloured. This he supposes to be
due to the colour -taste, developed by the constant presence
of bright flowers and fruits, being applied to the selection of
each variation towards brilliancy in their mates ; thus in time
producing the gorgeous and varied hues they now possess.
Mr. Allen maintains that "insects are bright where bright
flowers exist in numbers, and dull where flowers are rare or
inconspicuous ; " and he urges that " we can hardly explain this
wide coincidence otherwise than by supposing that a taste
for colour is produced through the constant search for food
among entomophilous blossoms, and that this taste has reacted
upon its possessors through the action of unconscious sexual
selection."

The examples Mr. Allen quotes of bright insects being
associated with bright flowers seem very forcible, but are
really deceptive or erroneous ; and quite as many cases could
be quoted which prove the very opposite. For example, in
the dense equatorial forests flowers are exceedingly scarce,
and there is no comparison with the amount of floral colour
to be met with in our temperate meadows, woods, and hill-
sides. The forests about Para in the lower Amazon are
typical in this respect, yet they abound with the most
gorgeously coloured butterflies, almost all of which frequent
the forest depths, keeping near the ground, where there is the
greatest deficiency of brilliant flowers. In contrast with this
let us take the Cape of Good Hope— the most flowery region
probably that exists upon the globe, — where the country
is a complete flower-garden of heaths, pelargoniums, mesembry-
anthemus, exquisite iridaceous and other bulbs, and numerous
flowering shrubs and trees ; yet the Cape butterflies are hardly
equal, either in number or variety, to those of any country
in South Europe, and are utterly insignificant when compared
with those of the comparatively flowerless forest-depths of
the Amazon or of New Guinea. Neither is there any relation
between the colours of other insects and their haunts. Few
1 The Colour Sense, chap. ix.

336 DARWINISM

are more gorgeous than some of the tiger-beetles and the
carabi, yet these are all carnivorous ; while many of the most
brilliant metallic buprestidas and longicorns are always found
on the bark of fallen trees. So with the humming-birds ;
their brilliant metallic tints can only be compared with metals
or gems, and are totally unlike the delicate pinks and purples,
yellows and reds of the majority of flowers. Again, the
Australian honey-suckers (Meliphagidse) are genuine flower-
haunters, and the Australian flora is more brilliant in colour
display than that of most tropical regions, yet these birds are,
as a rule, of dull colours, not superior on the average to our
grain-eating finches. Then, again, we have the grand pheasant
family, including the gold and the silver pheasants, the gorgeous
fire-backed and ocellated pheasants, and the resplendent pea-
cock, all feeding on the ground on grain or seeds or insects,
yet adorned with the most gorgeous colours.

There is, therefore, no adequate basis of facts for this theory
to rest upon, even if there were the slightest reason to believe
that not only birds, but butterflies and beetles, take any
delight in colour for its own sake, apart from the food-supply
of which it indicates the presence. All that has been proved or
that appears to be probable is, that they are able to perceive
differences of colour, and to associate each colour with the
particular flowers or fruits which best satisfy their wants.
Colour being in its nature diverse, it has been beneficial for
them to be able to distinguish all its chief varieties, as mani-
fested more particularly in the vegetable kingdom, and among
the different species of their own group ; and the fact that
certain species of insects shoAV some preference for a particular
colour may be explained by their having found flowers of
that colour to yield them a more abundant supply of nectar
or of pollen. In those cases in which butterflies frequent
flowers of their own colour, the habit may well have been
acquired from the protection it affords them.

It appears to me that, in imputing to insects and birds the
same love of colour for its own sake and the same aesthetic
tastes as we ourselves possess, Ave may be as far from the truth
as were those writers who held that the bee was a good mathe-
matician, and that the honeycomb was constructed throughout
to satisfy its refined mathematical instincts ; whereas it is now

xi THE SPECIAL COLOURS OF PLANTS 337

generally admitted to be the result of the simple principle of
economy of material applied to a primitive cylindrical cell.1

In studying the phenomena of colour in the organic world
we have been led to realise the wonderful complexity of the
adaptations which bring each species into harmonious relation
with all those which surround it, and which thus link together
the whole of nature in a network of relations of marvellous
intricacy. Yet all this is but, as it were, the outward show
and garment of nature, behind which lies the inner structure
— the framework, the vessels, the cells, the circulating fluids,
and the digestive and reproductive processes, — and behind
these again those mysterious chemical, electrical, and vital
forces which constitute what we term Life. These forces
appear to be fundamentally the same for all organisms, as
is the material of which all are constructed ; and we thus
find behind the outer diversities an inner relationship which
binds together the myriad forms of life.

Each species of animal or plant thus forms part of one
harmonious whole, carrying in all the details of its complex
structure the record of the long story of organic development ;
and it was with a truly inspired insight that our great philo-
sophical poet apostrophised the humble weed —

Flower in the crannied wall,

I pluck you out of the crannies,

I hold you here, root and all, in my hand,

Little flower — but if I could understand

What you are, root and all, and all in all,

I should know what God and man is.

1 See Origin of Species, sixth edition, p. 220.

CHAPTER XII

THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS

The facts to be explained — The conditions which have determined dis-
tribution— The permanence of oceans — Oceanic and continental areas
— Madagascar and New Zealand — -The teachings of the thousand-
fathom line — The distribution of marsupials — The distribution of
tapirs — Powers of dispersal as illustrated by insular organisms — Birds
and insects at sea — Insects at great altitudes — The dispersal of plants
— Dispersal of seeds by the wind — Mineral matter carried by the wind
— Objections to the theory of wind-dispersal answered — Explanation
of north temperate plants in the southern hemisphere — No proof of
glaciation in the tropics — Lower temperature not needed to explain
the facts — Concluding remarks.

The theory which we may now take as established — that all
the existing forms of life have been derived from other forms
by a natural process of descent with modification, and that
this same process has been in action during past geological
time — should enable us to give a rational account not only of
the peculiarities of form and structure presented by animals
and plants, but also of their grouping together in certain
areas, and their general distribution over the earth's surface.

In the absence of any exact knowledge of the facts of
distribution, a student of the theory of evolution might
naturally anticipate that all groups of allied organisms would
be found in the same region, and that, as he travelled farther
and farther from any given centre, the forms of life would
differ more and more from those which prevailed at the
starting-point, till, in the remotest regions to which he could
penetrate, he would find an entirely new assemblage of
animals and plants, altogether unlike those with which he was

ch. xn GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 339

familiar. He would also anticipate that diversities of climate
would always be associated with a corresponding diversity in
the forms of life.

Now these anticipations are to a considerable extent justi-
fied. Remoteness on the earth's surface is usually an indi-
cation of diversity in the fauna and flora, while strongly
contrasted climates are always accompanied by a considerable
contrast in the forms of life. But this correspondence is by
no means exact or proportionate, and the converse propositions
are often quite untrue. Countries which are near to each
other often differ radically in their animal and vegetable pro-
ductions ; while similarity of climate, together with moderate
geographical proximity, are often accompanied by marked
diversities in the prevailing forms of life. Again, while many
groups ^of animals — genera, families, and sometimes even
orders — are confined to limited regions, most of the families,
many genera, and even some species are found in every part
of the earth. An enumeration of a few of these anomalies will
better illustrate the nature of the problem we have to solve.

As examples of extreme diversity, notwithstanding geo-
graphical proximity, we may adduce Madagascar and Africa,
whose animal and vegetable productions are far less alike than
are those of Great Britain and Japan at the remotest ex-
tremities of the great northern continent ; while an equal, or
perhaps even a still greater, diversity exists between Australia
and New Zealand. On the other hand, Northern Africa and
South Europe, though separated by the Mediterranean Sea,
have faunas and floras which do not differ from each other
more than do the various countries of Europe. As a proof
that similarity of climate and general adaptability have had
but a small part in determining the forms of life in each
country, we have the fact of the enormous increase of rabbits
and pigs in Australia and New Zealand, of horses and cattle
in South America, and of the common sparrow in North
America, though in none of these cases are the animals
natives of the countries in which they thrive so well.
And lastly, in illustration of the fact that allied forms are
not always found in adjacent regions, we have the tapirs,
which are found only on opposite sides of the globe, in
tropical America and the Malayan Islands ; the camels of

340 DARWINISM chap.

the Asiatic deserts, whose nearest allies are the llamas
and alpacas of the Andes; and the marsupials, only found
in Australia and on the opposite side of the globe, in
America. Yet, again, although mammalia may be said to
be universally distributed over the globe, being found abund-
antly on all the continents and on a great many of the larger
islands, yet they are entirely wanting in New Zealand, and in
a considerable number of other islands which are, nevertheless,
perfectly able to support them Avhen introduced.

Now most of these difficulties can be solved by means of
well-known geographical and geological facts. When the pro-
ductions of remote countries resemble each other, there is
almost always continuity of land with similarity of climate
between them. When adjacent countries differ greatly in
their productions, we find them separated by a sea or strait
whose great depth is an indication of its antiquity or per-
manence. When a group of animals inhabits two coun-
tries or regions separated by wide oceans, it is found that
in past geological times the same group was much more
widely distributed, and may have reached the countries it
inhabits from an intermediate region in which it is now extinct.
We know, also, that countries now united by land were
divided by arms of the sea at a not very remote epoch ; while
there is good reason to believe that others now entirely
isolated by a broad expanse of sea were formerly united and
formed a single land area. There is also another important
factor to be taken account of in considering how animals and
plants have acquired their present peculiarities of distribution,
— changes of climate. We know that quite recently a glacial
epoch extended over much of what are now the temperate
regions of the northern hemisphere, and that consequently
the organisms which inhabit those parts must be, com-
paratively speaking, recent immigrants from more southern
lands. But it is a yet more important fact that, down to
middle Tertiary times at all events, an equable temperate
climate, with a luxuriant vegetation, extended to far within
the arctic circle, over what are now barren wastes, covered
for ten months of the year with snow and ice. The arctic
zone has, therefore, been in past times capable of supporting
almost all the forms of life of our temperate regions ; and we

xn GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 341

must take account of this condition of things whenever we
have to speculate on the possible migrations of organisms
between the old and new continents.

The Conditions which have determined Distribution.

When we endeavour to explain in detail the facts of the
existing distribution of organic beings, we are confronted by
several preliminary questions, upon the solution of which will
depend our treatment of the phenomena presented to us.
Upon the theory of descent which we have adopted, all the
different species of a genus, as well as all the genera which
compose a family or higher group, have descended from some
common ancestor, and must therefore, at some remote epoch,
have occupied the same area, from which their descendants
have spread to the regions they now inhabit. In the numerous
cases in 'which the same group now occupies countries separated
by oceans or seas, by lofty mountain-chains, by wide deserts,
or by inhospitable climates, we have to consider how the
migration which must certainly have taken place has been
effected. It is possible that during some portion of the time
which has elapsed since the origin of the group the inter-
posing barriers have not been in existence ; or, on the other
hand, the particular organisms we are dealing with may have
the power of overpassing the barriers, and thus reaching their
present remote dwelling-places. As this is really the funda-
mental question of distribution on which the solution of all
its more difficult problems depends, Ave have to inquire, in the
first place, what is the nature of, and what are the limits to, the
changes of the earth's surface, especially during the Tertiary
and latter part of the Secondary periods, as it was during those
periods that most of the existing types of the higher animals
and plants came into existence ; and, in the next place, Avhat
are the extreme limits of the powers of dispersal possessed by
the chief groups of animals and plants. We will first consider
the question of barriers, more especially those formed by seas

and oceans.

The Permanence of Oceans.

It was formerly a very general belief, even amongst
geologists, that the great features of the earth's surface, no less
than the smaller ones, were subject to continual mutations,

342 DARWINISM

and that during the course of known geological time the
continents and great oceans had again and again changed
places with each other. Sir Charles Lyell, in the last edition
of his Principles of Geology (1872), said: "Continents, there-
fore, although permanent for whole geological epochs, shift
their positions entirely in the course of ages ; " and this may
be said to have been the orthodox opinion down to the very
recent period when, by means of deep-sea soundings, the nature
of the ocean bottom was made known. The first person to
throw doubt on this view appears to have been the veteran
American geologist, Professor Dana. In 1849, in the Eeport
of Wilke's Exploring Expedition, he adduced the argument
against a former continent in the Pacific during the Tertiary
period, from the absence of all native quadrupeds. In 1856,
in articles in the American Journal, he discussed the develop-
ment of the American continent, and argued for its general
permanence ; and in his Manual of Geology in 1863 and later
editions, the same views were more fully enforced and were
latterly applied to all continents. Darwin, in his Journal of
Researches, published in 1845, called attention to the fact that
all the small islands far from land in the Pacific, Indian, and
Atlantic Oceans are either of coralline or volcanic formation.
He excepted, however, the Seychelles and St. Paul's rocks ;
but the former have since been shown to be no exception, as
they consist entirely of coral rock ; and although Darwin
himself spent a few hours on St. Paul's rocks on his outward
voyage in the Beagle, and believed he had found some
portions of them to be of a " cherty," and others of a
" felspathic " nature, this also has been shown to be erroneous,
and the careful examination of the rocks by the Abbe Eenard
clearly proves them to be wholly of volcanic origin.1 We
have, therefore, at the present time, absolutely no exception
whatever to the remarkable fact that all the oceanic islands of
the globe are either of volcanic or coral formation ; and there
is, further, good reason to believe that those of the latter class
in every case rest upon a volcanic foundation.

In his Origin of Species, Darwin further showed that no
true oceanic island had any native mammals or batrachia

1 See A. Agassiz, Three Cruises of the Blake (Cambridge, Mass., 18SS),
vol. i. p. 127, footnote.

xii GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 343

when first discovered, this fact constituting the test of the
class to which an island belongs ; whence he argued that none
of them had ever been connected with continents, but all had
originated in mid-ocean. These considerations alone render
it almost certain that the areas now occupied by the great
oceans have never, during known geological time, been
occupied by continents, since it is in the highest degree im-
probable that every fragment of those continents should have
completely disappeared, and have been replaced by volcanic
islands rising out of profound oceanic abysses ; but recent
research into the depth of the oceans and the nature of the
deposits now forming on their floors, adds greatly to the
evidence in this direction, and renders it almost a certainty
that they represent very ancient if not primaeval features of
the earth's surface. A very brief outline of the nature of this
evidence will be now given.

The researches of the Challenger expedition into the
nature of the sea-bottom show, that the whole of the land
debris brought down by rivers to the ocean (with the ex-
ception of pumice and other floating matter), is deposited
comparatively near to the shores, and that the fineness of the
material is an indication of the distance to which it has been
carried. Everything in the nature of gravel and sand is laid
down within a very few miles of land, only the finer muddy
sediments being carried out for 20 or 50 miles, and
the very finest of all, under the most favourable conditions,
rarely extending beyond 150, or at the utmost, 300 miles
from land into the deep ocean.1 Beyond these distances, and
covering the entire ocean floor, are various oozes formed wholly
from the debris of marine organisms ; while intermingled with
these are found various volcanic products which have been
either carried through the air or floated on the surface, and a
small but perfectly recognisable quantity of meteoric matter.
Ice-borne rocks are also found abundantly scattered over the
ocean bottom within a definite distance of the arctic and
antarctic circles, clearly marking out the limit of floating ice-
bergs in recent geological times.

1 Even the extremely fine Mississippi mud is nowhere found beyond a
hundred miles from the mouths of the river in the Gulf of Mexico (A. Agassiz,
Three Cruises of the Blake, vol. i. p. 128).

344 DARWINISM

Now the whole series of marine stratified rocks, from the
earliest Palaeozoic to the most recent Tertiary beds, consist of
materials closely corresponding to the land debris now being
deposited within a narrow belt round the shores of all con-
tinents ; while no rocks have been found which can be identified
with the various oozes now forming in the deep abysses of the
ocean. It follows, therefore, that all the geological formations
have been formed in comparatively shallow water, and always
adjacent to the continental land of the period. The great
thickness of some of the formations is no indication of a deep
sea, but only of slow subsidence during the time that the
deposition was in progress. This view is now adopted by
many of the most experienced geologists, especially by Dr.
Archibald Geikie, Director of the Geological Survey of Great
Britain, who, in his lecture on " Geographical Evolution," says :
"From all this evidence we may legitimately conclude that
the present land of the globe, though consisting in great
measure of marine formations, has never lain under the deep
sea ; but that its site must always have been near land. Even
its thick marine limestones are the deposits of comparatively
shallow water." x

But besides these geological and physical considerations,
there is a mechanical difficulty in the way of repeated
change of position of oceans and continents which has not
yet received the attention it deserves. According to the
recent careful estimate by Mr. John Murray, the land area
of the globe is to the water area as "28 to "72. The mean
height of the land above sea-level is 2250 feet, while the
mean depth of the ocean is 14,640 feet. Hence the bulk
of dry land is 23,450,000 cubic miles, and that of the waters
of the ocean 323,800,000 cubic miles; and it follows that if
the whole of the solid matter of the earth's surface were
reduced to one level, it would be everywhere covered by an
ocean about two miles deep. The accompanying diagram will
serve to render these figures more intelligible. The length of
the sections of land and ocean are in the proportion of their
respective areas, while the mean height of the land and the
mean depth of the ocean are exhibited on a greatly increased

1 I have given a full summary of the evidence for the permanence of
oceanic and continental areas in my Island Life, chap. vi.

xii GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 345

vertical scale. If we considered the continents and their
adjacent oceans separately they would differ a little, but not
very materially, from this diagram ; in some cases the propor-
tion of land to ocean would be a little greater, in others a little
less.

Now, if we try to imagine a process of elevation and
depression by which the sea and land shall completely change
places, we shall be met by insuperable difficulties. We must,
in the first place, assume a general equality between ele-
vation and subsidence during any given period, because if
the elevation over any extensive continental area were not
balanced by some subsidence of approximately equal amount,

Diagram of proporti

mate meao height of Land and depth of Oceans.

Land

Area. -28 of area
of Globe.

—

Ocean
Area. -72 of area of Globe.

Fig. 32.

an unsupported hollow would be left under the earth's crust.
Let us now suppose a continental area to sink, and an adjacent
oceanic area to rise, it will be seen that the greater part of
the land will disappear long before the new land has approached
the surface of the ocean. This difficulty will not be removed
by supposing a portion of a continent to subside, and the
immediately adjacent portion of the ocean on the other side
of the continent to rise, because in almost every case we find
that within a comparatively short distance from the shores of
all existing continents, the ocean floor sinks rapidly to a depth
of from 2000 to 3000 fathoms, and maintains a similar depth,
generally speaking, over a large portion of the oceanic areas.
In order, therefore, that any area of continental extent be
upraised from the great oceans, there must be a subsidence of
a land area five or six times as great, unless it can be shown
that an extensive elevation of the ocean floor up to and far

346 DARWINISM

above the surface could occur without an equivalent depression
elsewhere. The fact that the waters of the ocean are sufficient
to cover the whole globe to a depth of two miles, is alone
sufficient to indicate that the great ocean basins are permanent
features of the earth's surface, since any process of alterna-
tion of these with the land areas would have been almost
certain to result again and again in the total disappearance of
large portions, if not of all, of the dry land of the globe. But
the continuity of terrestrial life since the Devonian and Car-
boniferous periods, and the existence of very similar forms in
the corresponding deposits of every continent — as well as the
occurrence of sedimentary rocks, indicating the proximity of
land at the time of their deposit, over a large portion of the
surface of all the continents, and in every geological period — ■
assure us that no such disappearance has ever occurred.

Oceanic and Continental Areas.

When we speak of the permanence of oceanic and conti-
nental areas as one of the established facts of modern research,
we do not mean that existing continents and oceans have
always maintained the exact areas and outlines that they now
present, but merely, that while all of them have been under-
going changes in outline and extent from age to age, they
have yet maintained substantially the same positions, and
have never actually changed places with each other. There
are, moreover, certain physical and biological facts which
enable us to mark out these areas with some confidence.

We have seen that there are a large number of islands
which may be classed as oceanic, because they have never
formed parts of continents, but have originated in mid-ocean,
and have derived their forms of life by migration across the
sea. Their peculiarities are seen to be very marked in com-
parison with those islands which there is good reason to
believe are really fragments of more extensive land areas, and
are hence termed "continental." These continental islands
consist in every case of a variety of stratified rocks of various
ages, thus corresponding closely with the usual structure of
continents ; although many of the islands are small like
Jersey or the Shetland Islands, or far from continental
land like the Falkland Islands or New Zealand. They all

xii GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 347

contain indigenous mammalia or batrachia, and generally a
much greater variety of birds, reptiles, insects, and plants,
than do the oceanic islands. From these various character-
istics we conclude that they have all once formed parts of
continents, or at all events of much larger land areas, and have
become isolated, either by subsidence of the intervening land
or by the effects of long-continued marine denudation.

Now, if we trace the thousand-fathom line around all our
existing continents we find that, with only two exceptions,
every island which can be classed as " continental " falls
within this line, while all that lie beyond it have the un-
doubted characteristics of " oceanic " islands. We, therefore,
conclude that the thousand-fathom line marks out, approxi-
mately, the "continental area," — that is, the limits within
which continental development and change throughout known
geological time have gone on. There may, of course, have
been some extensions of land beyond this limit, while some
areas Avithin it may always have been ocean ; but so far as
we have any direct evidence, this line may be taken to mark
out, approximately, the most probable boundary between the
"continental area," which has always consisted of land and
shallow sea in varying proportions, and the great oceanic
basins, within the limits of which volcanic activity has been
building up numerous islands, but whose profound depths
have apparently undergone little change.

Madagascar and New Zealand.

The two exceptions just referred to are Madagascar and
New Zealand, and all the evidence goes to show that in these
cases the land connection with the nearest continental area
was very remote in time. The extraordinary isolation of the
productions of Madagascar — almost all the most characteristic
forms of mammalia, birds, and reptiles of Africa being
absent from it — renders it certain that it must have been
separated from that continent very early in the Tertiary, if
not as far back as the latter part of the Secondary period ;
and this extreme antiquity is indicated by a depth of
considerably more than a thousand fathoms in the Mozam-
bique Channel, though this deep portion is less than a
hundred miles wide between the Comoro Islands and the main-

348 DARWINISM

land.1 Madagascar is the only island on the globe with a fairly-
rich mammalian fauna which is separated from a continent by
a depth greater than a thousand fathoms ; and no other island
presents so many peculiarities in these animals, or has pre-
served so many lowly organised and archaic forms. The
exceptional character of its productions agrees exactly with its
exceptional isolation by means of a very deep arm of the sea.
New Zealand possesses no known mammals and only a
single species of batrachian ; but its geological structure is
perfectly continental. There is also much evidence that it
does possess one mammal, although no specimens have been
yet obtained.2 Its reptiles and birds are highly peculiar and
more numerous than in any truly oceanic island. Now the
sea which directly separates New Zealand from Australia is
more than 2000 fathoms deep, but in a north-west direction
there is an extensive bank under 1000 fathoms, extending to
and including Lord Howe's Island, while north of this are
other banks of the same depth, approaching towards- a sub-
marine extension of Queensland on the one hand, and New
Caledonia on the other, and altogether suggestive of a land
union with Australia at some very remote period. Now the
peculiar relations of the New Zealand fauna and flora with
those of Australia and of the tropical Pacific Islands to the
northward indicate such a connection, probably during the
Cretaceous period ; and here, again, we have the exceptional
depth of the dividing sea and the form of the ocean bottom
according well with the altogether exceptional isolation of
New Zealand, an isolation which has been held by some
naturalists to be great enough to justify its claim to be one
of the primary Zoological Regions.

The Teachings of Hie Thousand-Fathom Line.

If now we accept the annexed map as showing us approxi-
mately how far beyond their present limits our continents may

1 For a full account of the peculiarities of the Madagascar fauna, see my
Island Life, chap. xix.

2 See Island Life, p. 446, and the whole of chaps, xxi. xxii. More
recent soundings have shown that the Map at p. 443, as well as that of the
Madagascar group at p. 387, are erroueous, the ocean around Norfolk Island
and in the Straits of Mozambique being more than 1000 fathoms deep.
The general argument is, however, unaffected.

40 ;iu

ee'n land

yiaufes5

40 30

xii GEOGRAPHICAL DISTRIBUTION" OF ORGANISMS 349

have extended during any portion of the Tertiary and Secondary
periods, we shall obtain a foundation of inestimable value for
our inquiries into those migrations of animals and plants
during past ages which have resulted in their present peculi-
arities of distribution. We see, for instance, that the South
American and African continents have always been separated
by nearly as wide an ocean as at present, and that whatever
similarities there may be in their productions must be due to
the similar forms having been derived from a common origin
in one of the great northern continents. The radical difference
between the higher forms of life of the two continents accords
perfectly with their permanent separation. If there had been
any direct connection between them during Tertiary times, we
should hardly have found the cleep-seatecl differences between
the Quadrumana of the two regions — no family even being-
common to both ; nor the peculiar Insectivora of the one
continent, and the equally peculiar Edentata of the other.
The very numerous families of birds quite peculiar to one or
other of these continents, many of which, by their structural
isolation and varied development of generic and specific forms,
indicate a high antiquity, equally suggest that there has been
no near approach to a land connection during the same epoch.
Looking to the two great northern continents, we see indica-
tions of a possible connection between them both in the North
Atlantic and the North Pacific oceans ; and when we remember
that from middle Tertiary times backward — so far as we know
continuously to the earliest Palaeozoic epoch — a temperate and
equable climate, with abundant woody vegetation, prevailed
up to and within the arctic circle, we see what facilities
may have been afforded for migration from one continent
to the other, sometimes between America and Europe, some-
times between America and Asia. Admitting these highly
probable connections, no bridging of the Atlantic in more
southern latitudes (of which there is not a particle of evidence)
will have been necessary to account for all the intermigration
that has occurred between the two continents. If, on the
other band, we remember how long must have been the route,
and how diverse must always have been the conditions be-
tween the more northern and the more southern portions of
the American and Euro -Asiatic continents, we shall not be

350 DARWINISM

surprised that many widespread forms in either continent
have not crossed into the other ; and that while the skunks
(Mephitis), the pouched rats (Saccomyidae), and the turkeys
(Meleagris) are confined to America, the pigs and the hedge-
hogs, the true flycatchers and the pheasants are found only
in the Euro-Asiatic continent. But, just as there have been
periods which facilitated intermigration between America and
the Old World, there have almost certainly been periods,
perhaps of long duration even geologically, when these con-
tinents have been separated by seas as wide as, or even wider
than, those of the present day ; and thus may be explained
such curious anomalies as the origination of the camel-tribe in
America,, and its entrance into Asia in comparatively recent
Tertiary times, while the introduction of oxen and bears into
America from the Euro- Asiatic continent appears to have been
equally recent.1

We shall find on examination that this view of the general
permanence of the oceanic and continental areas, with constant
minor fluctuations of land and sea over the whole extent of
the latter, enables us to understand, and offer a rational
explanation of, most of the difficult problems of geographical
distribution ; and further, that our power of doing this is in
direct proportion to our acquaintance with the distribution of
fossil forms of life during the Tertiary period. We must, also,
take due note of many other facts of almost equal importance
for a due appreciation of the problems presented for solution,
the most essential being, the various powers of dispersal
possessed by the different groups of animals and plants, the
geological antiquity of the species and genera, and the width
and depth of the seas which separate the countries they
inhabit. A few illustrations will now be given of the way in
which these branches of knowledge enable us to deal with the
difficulties and anomalies that present themselves.

The Distribution of Marsupials.

This singular and lowly organised type of mammals con-
stitutes almost the sole representative of the class in Australia

1 For some details of these migrations, see the author's Geographical
Distribution of Animals, vol. i. p. 140 ; also Heilprin's Geographical and
Geological Distribution of Animals.

xii GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 351

and New Guinea, while it is entirely unknown in Asia, Africa,
or Europe. It reappears in America, where several species of
opossums are found; and it was long thought necessary to postu-
late a direct southern connection of these distant countries,
in order to account for this curious fact of distribution. When,
however, we look to what is known of the geological history
of the marsupials the difficulty vanishes. In the Upper Eocene
deposits of Western Europe the remains of several animals
closely allied to the American opossums have been found ;
and as, at this period, a very mild climate prevailed far up
into the arctic regions, there is no difficulty in supposing that
the ancestors of the group entered America from Europe or
Northern Asia during early Tertiary times.

But we must go much further back for the origin of the
Australian marsupials. All the chief types of the higher
mammalia were in existence in the Eocene, if not in the preceding
Cretaceous period, and as we find none of these in Australia,
that country must have been finally separated from the Asiatic
continent during the Secondary or Mesozoic period. Now
during that period, in the Upper and the Lower Oolite and
in the still older Trias, the jaw-bones of numerous small
mammalia have been found, forming eight distinct genera,
which are believed to have been either marsupials or some
allied lowly forms. In North America also, in beds of the
Jurassic and Triassic formations, the remains of an equally great
variety of these small mammalia have been discovered ; and
from the examination of more than sixty specimens, belonging
to at least six distinct genera, Professor Marsh is of opinion
that they represent a generalised type, from which the more
specialised marsupials and insectivora were developed.

From the fact that very similar mammals occur both in
Europe and America at corresponding periods, and in beds
which represent a long succession of geological time, and that
during the whole of this time no fragments of any higher
forms have been discovered, it seems probable that both the
northern continents (or the larger portion of their area) were
then inhabited by no other mammalia than these, with
perhaps other equally low types. It was, probably, not later
than the Jurassic age when some of these primitive marsu-
pials were able to enter Australia, where they have since

352 DARWINISM

remained almost completely isolated ; and, being free from the
competition of higher forms, they have developed into the
great variety of types we now behold there. These occupy the
place, and have to some extent acquired the form and structure
of distinct orders of the higher mammals — the rodents, the
insectivora, and the carnivora, — while still preserving the
essential characteristics and lowly organisation of the mar-
supials. At a much later period — probably in late Tertiary
times — the ancestors of the various species of rats and mice
which now abound in Australia, and which, with the aerial bats,
constitute its only forms of placental mammals, entered the
country from some of the adjacent islands. For this purpose
a land connection was not necessary, as these small creatures
might easily be conveyed among the branches or in the crevices
of trees uprooted by floods and carried down to the sea, and
then floated to a shore many miles distant. That no actual land
connection with, or very close approximation to, an Asiatic
island has occurred in recent times, is sufficiently proved by
the fact that no squirrel, pig, civet, or other widespread
mammal of the Eastern hemisphere has been able to reach the
Australian continent.

The Distribution of Tapirs.

These curious animals form one of the puzzles of geographi-
cal distribution, being now confined to two very remote regions
of the globe — the Malay Peninsula and adjacent islands of
Sumatra and Borneo, inhabited by one species, and tropical
America, where there are three or four species, ranging from
Brazil to Ecuador and Guatemala. If we considered these
living forms only, we should be obliged to speculate on
enormous changes of land and sea in order that these tropical
animals might have passed from one country to the other. But
geological discoveries have rendered all such hypothetical
changes unnecessary. During Miocene and Pliocene times
tapirs abounded over the whole of Europe and Asia, their
remains having been found in the tertiary deposits of France,
India, Burmah, and China. In both North and South
America fossil remains of tapirs occur only in caves and de-
posits of Post-Pliocene age, showing that they are compara-
tively recent immigrants into that continent. They perhaps

xii GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 353

entered by the route of Kamchatka and Alaska, where the
climate, even now so much milder and more equable than on
the north-east of America, might have been warm enough in
late Pliocene times to have allowed the migration of these
animals. In Asia they were driven southwards by the
competition of numerous higher and more powerful forms,
but have found a last resting-place in the swampy forests of
the Malay region.

What these Facts Prove.

Now these two cases, of the marsupials and the tapirs,
are in the highest degree instructive, because they show
us that, without any hypothetical bridging of deep oceans,
and with only such changes of sea and land as are indi-
cated by the extent of the comparatively shallow seas
surrounding and connecting the existing continents, we are
able to account for the anomaly of allied forms occurring
only in remote and widely separated areas. These examples
really constitute crucial tests, because, of all classes of animals,
mammalia are least able to surmount physical barriers. They
are obviously unable t.o pass over wide arms of the sea,
while the necessity for constant supplies of food and water
renders sandy deserts or snow -clad plains equally impass-
able. Then, again, the peculiar kinds of food on which
alone many of them can subsist, and their liability to the
attacks of other animals, put a further check upon their
migrations. In these respects almost all other organisms
have great advantages over mammals. Birds can often fly
long distances, and can thus cross arms of the sea, deserts, or
mountain ranges ; insects not only fly, but are frequently
carried great distances by gales of wind, as shown by the
numerous cases of their visits to ships hundreds of miles from
land. Eeptiles, though slow of movement, have advantages in
their greater capacity for enduring hunger or thirst, their power
of resisting cold or drought in a state of torpidity, and they
have also some facilities for migration across the sea by means
of their eggs, which may be conveyed in crevices of timber or
among masses of floating vegetable matter. And when we
come to the vegetable kingdom, the means of transport are
at their maximum, numbers of seeds having special adaptations

2 A

354 DARWINISM

for being carried by mammalia or birds, and for floating in the
water, or through the air, while many are so small and so
light that there is practically no limit to the distances they
may be carried by gales and hurricanes.

We may, therefore, feel quite certain that the means of dis-
tribution that have enabled the larger mammalia to reach the
most remote regions from a common starting-point, will be at
least as efficacious, and usually far more efficacious, with all other
land animals and plants ; and if in every case the existing
distribution of this class can be explained on the theory of
oceanic and continental permanence, with the limited changes
of sea and land already referred to, no valid objections can be
taken against this theory founded on anomalies of distribution
in other orders. Yet nothing is more common than for
students of this or that group to assert that the theory of
oceanic permanence is quite inconsistent with the distribution
of its various species and genera. Because a few Indian
genera and closely allied species of birds are found in Mada-
gascar, a land termed " Lemuria " has been supposed to have
united the two countries during a comparatively recent
geological epoch ; while the similarity of fossil plants and
reptiles, from the Permian and Miocene formations of India
and South Africa, has been adduced as further evidence of this
connection. But there are also genera of snakes, of insects,
and of plants, common to Madagascar and South America
only, which have been held to necessitate a direct land
connection between these countries. These views evidently
refute themselves, because any such land connections must
have led to a far greater similarity in the productions of
the several countries than actually exists, and would besides
render altogether inexplicable the absence of all the chief
types of African and Indian mammalia from Madagascar, and
its marvellous individuality in every department of the organic
world.1

Powers of Dispersal as illustrated by Insular Organisms.

Having arrived at the conclusion that our existing oceans
have remained practically unaltered throughout the Tertiary and
Secondary periods of geology, and that the distribution of the

1 For a full discussion of tins question, see Island Life, pp. 390-420.

xii GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 355

mammalia is such as might have been brought about by their
known powers of dispersal, and by such changes of land and
sea as have probably or certainly occurred, we are, of course,
restricted to similar causes to explain the much wider and
sometimes more eccentric distribution of other classes of
animals and of plants. In doing so, we have to rely partly on
direct evidence of dispersal, afforded by the land organisms
that have been observed far out at sea, or which have taken
refuge on ships, as well as by the periodical visitants to remote
islands ; but very largely on indirect evidence, afforded by
the frequent presence of certain groups on remote oceanic
islands, which some ancestral forms must, therefore, have
reached by transmission across the ocean from distant lands.

Birds.

These-" vary much in their powers of flight, and their
capability of traversing wide seas and oceans. Many
swimming and wading birds can continue long on the wing,
fly swiftly, and have, besides, the power of resting safely
on the surface of the water. These would hardly be limited
by any width of ocean, except for the need of food ; and many
of them, as the gulls, petrels, and divers, find abundance of
food on the surface of the sea itself. These groups have a
wide distribution across the oceans ; while waders — especially
plovers, sandpipers, snipes, and herons — are equally cos-
mopolitan, travelling along the coasts of all the continents,
and across the narrow seas which separate them. Many of
these birds seem unaffected by climate, and as the organisms
on which they feed are equally abundant on arctic, temperate,
and tropical shores, there is hardly any limit to the range
even of some of the species.

Land-birds are much more restricted in their range, owing
to their usually limited powers of flight, their inability to rest
on the surface of the sea or to obtain food from it, and their
greater specialisation, which renders them less able to main-
tain themselves in the new countries they may occasionally
reach. Many of them are adapted to live only in woods, or
in marshes, or in deserts ; they need particular kinds of food
or a limited range of temperature ; and they are adapted to
cope only with the special enemies or the particular group of

356 DARWINISM

competitors among which they have been developed. Such
birds as these may pass again and again to a new country, but
are never able to establish themselves in it ; and it is this
organic barrier, as it is termed, rather than any physical
barrier, which, in many cases, determines the presence of a
species in one area and its absence from another. We must
always remember, therefore, that, although the presence of a
species in a remote oceanic island clearly proves that its
ancestors must at one time have found their way there, the
absence of a species does not prove the contrary, since it also
may have reached the island, but have been unable to main-
tain itself, owing to the inorganic or organic conditions not
being suitable to it. This general principle applies to all
classes of organisms, and there are many striking illustrations
of it. In the Azores there are eighteen species of land-birds
which are permanent residents, but there are also several
others which reach the islands almost every year after great
storms, but have never been able to establish themselves. In
Bermuda the facts are still more striking, since there are only
ten species of resident birds, while no less than twenty other
species of land -birds and more than a hundred species of
waders and aquatics are frequent visitors, often in great
numbers, but are never able to establish themselves. On
the same principle we account for the fact that, of the many
continental insects and birds that have been let loose, or
have escaped from confinement, in this country, hardly
one has been able to maintain itself, and the same pheno-
menon is still more striking in the case of plants. Of the
thousands of hardy plants which grow easily in our gardens,
very few have ever run wild, and when the experiment
is purposely tried it invariably fails. Thus A. de Candolle
informs us that several botanists of Paris, Geneva, and
especially of Montpellier, have sown the seeds of many
hundreds of species of exotic hardy plants, in what appeared
to be the most favourable situations, but that in hardly a
single case has any one of them become naturalised.1 Still
more, then, in plants than in animals the absence of a species
does not prove that it has never reached the locality, but
merely that it has not been able to maintain itself in com-
1 Geographic Botanique, p. 79S.

xii GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 357

petition with the native productions. In other cases, as we
have seen, facts of an exactly opposite nature occur. The rat,
the pig, and the rabbit, the water-cress, the clover, and many
other plants, when introduced into New Zealand, nourish
exceedingly, and even exterminate their native competitors ;
so that in these cases we may feel sure that the species in
question did not exist in New Zealand simply because
they had been unable to reach that country by their natural
means of dispersal. I will now give a few cases, in addition
to those recorded in my previous works, of birds and insects
which have been observed far from any land.

Birds and Insects at Sea.

Captain D. Fullarton of the ship Tirnaru recorded in his
log the occurrence of a great number of small land-birds about
the ship on 15th March 1886, when in Lat. 48° 31' N., Long.
8° 16' W. He says : "A great many small land-birds about us ;
put about sixty into a coop, evidently tired out." And two
days later, 17th March, "Over fifty of the birds cooped on
15th died, though fed. Sparrows, finches, water- wagtails, two
small birds, name unknown, one kind like a linnet, and a large
bird like a starling. In all there have been on board over
seventy birds, besides some that hovered about us for some
time and then fell into the sea exhausted." Easterly winds
and severe weather were experienced at the time.1 The spot
where this remarkable flight of birds was met with is about
160 miles due west of Brest, and this is the least distance the
birds must have been carried. It is interesting to note that
the position of the ship is nearly in the line from the English
and French coasts to the Azores, where, after great storms, so
many bird stragglers arrive annually. These birds were prob-
ably blown out to sea during their spring migration along the
south coast of England to Wales and Ireland. During the
autumnal migration, however, great flocks of birds — especially
starlings, thrushes, and fieldfares — have been observed every
year flying out to sea from the west coast of Ireland, almost
the whole of which must perish. At the Nash Lighthouse, in
the Bristol Channel on the coast of Glamorganshire, an enormous
number of small birds were observed on 3d September, includ-
1 Nature, 1st April 1886.

358 DARWINISM

ing nightjars, buntings, white-throats, willow-wrens, cuckoos,
house -sparrows, robins, wheatears, and blackbirds. These
had probably crossed from Somersetshire, and had they been
caught by a storm the larger portion of them must have been
blown out to sea.l

These facts enable us to account sufficiently well for the
birds of oceanic islands, the number and variety of which are
seen to be proportionate to their facilities for reaching the
island and maintaining themselves in it. Thus, though more
birds yearly reach Bermuda than the Azores, the number of
residents in the latter islands is much larger, due to the
greater extent of the islands, their number, and their more
varied surface. In the Galapagos the land-birds are still more
numerous, due in part to their larger area and greater proxi-
mity to the continent, but chiefly to the absence of storms,
so that the birds which originally reached the islands have
remained long isolated and have developed into many closely
allied species adapted to the special conditions. All the
species of the Galapagos but one are peculiar to the islands,
while the Azores possess only one peculiar species, and
Bermuda none — a fact which is clearly due to the continual
immigration of fresh individuals keeping up the purity of
the breed by intercrossing. In the Sandwich Islands, which
are extremely isolated, being more than 2000 miles from
any continent or large island, we have a condition of things
similar to what prevails in the Galapagos, the land -birds,
eighteen in number, being all peculiar, and belonging, except
one, to peculiar genera. These birds have probably all
descended from three or four original types which reached
the islands at some remote period, probably by means of
intervening islets that have since disappeared. In St. Helena
we have a degree of permanent isolation which has pre-
vented any land-birds from reaching the island ; for although
its distance from the continent, 1100 miles, is not so great
as in the case of the Sandwich Islands, it is situated in an
ocean almost entirely destitute of small islands, while its
position within the tropics renders it free from violent storms.
Neither is there, on the nearest part of the coast of Africa,
a perpetual stream of migrating birds like that which

1 Report of the Brit. Assoc. Committee on Migration of Birds during 1886.

xii GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 359

supplies the innumerable stragglers which every year reach
Bermuda and the Azores.

Insects.

Winged insects have been mainly dispersed in the same
way as birds, by their power of flight, aided by violent or long-
continued winds. Being so small, and of such low specific
gravity, they are occasionally carried to still greater distances ;
and thus no islands, however remote, are altogether without
them. The eggs of insects, being often deposited in borings
or in crevices of timber, may have been conveyed long
distances by floating trees, as may the larvae of those species
which feed on wood. Several cases have been published of
insects coming on board ships at great distances from land ;
and Darwin records having caught a large grasshopper when
the ship was 370 miles from the coast of Africa, whence the
insect had probably come.

In the Entomologists' Monthly Magazine for June 1885, Mr.
MacLachlan has recorded the occurrence of a swarm of moths
in the Atlantic ocean, from the log of the ship Pleione.
The vessel was homeward bound from New Zealand, and in
Lat. 6° 47' N., Long. 32° 50' W., hundreds of moths appeared
about the ship, settling in numbers on the spars and rigging.
The wind for four days previously had been very light from
north, north-west, or north-east, and sometimes calm. The north-
east trade wind occasionally extends to the ship's position at
that time of year. The captain adds that "frequently, in
that part of the ocean, he has had moths and butterflies
come on board." The position is 960 miles south-west of
the Cape Verde Islands, and about 440 north-east of the
South American coast. The specimen preserved is Deiopeia
pulchella, a very common species in dry localities in the
Eastern tropics, and rarely found in Britain, but, Mr. Mac-
Lachlan thinks, not found in South America. They must
have come, therefore, from the Cape Verde Islands, or from
some parts of the African coast, and must have traversed
about a thousand miles of ocean with the assistance, no doubt,
of a strong north-east trade wind for a great part of the distance.
In the British Museum collection there is a specimen of the
same moth caught at sea during the voyage of the Rattlesnake,

360 DARWINISM

in Lat. 6° 1ST., Long. 22|° W., being between the former position
and Sierra Leone, thus rendering it probable that the moths
came from that part of the African coast, in which case the
swarm encountered by the Pleione must have travelled more
than 1200 miles.

A similar case was recorded by Mr. F. A. Lucas in the
American periodical Science of 8th April 1887. He states
that in 1870 he met with numerous moths of many species
while at sea in the South Atlantic (Lat. 25° S., Long. 24° W.),
about 1000 miles from the coast of Brazil. As this position
is just beyond the south-east trades, the insects may have been
brought from the land by a westerly gale. In the Zoologist
(1864, p. 8920) is the record of a small longicorn beetle which
flew on board a ship 500 miles off the west coast of Africa.
Numerous other cases are recorded of insects at less distances
from land, and, taken in connection with those already given,
they are sufficient to show that great numbers must be con-
tinually carried out to sea, and that occasionally they are able
to reach enormous distances. But the reproductive powers of
insects are so great that all we require, in order to stock a
remote island, is that some few specimens shall reach it even
once in a century, or once in a thousand years.

Insects at great Altitudes.

Equally important is the proof we possess that insects are
often carried to great altitudes by upward currents of air.
Humboldt noticed them up to heights of 15,000 and 18,000
feet in South America, and Mr. Albert Midler has collected many
interesting cases of the same character in Europe.1 A moth
(Plusia gamma) has been found on the summit of Mont Blanc ;
small hymenoptera and moths have been seen on the Pyrenees
at a height of 11,000 feet, while numerous flies and beetles,
some of considerable size, have been caught on the glaciers
and snow- fields of various parts of the Alps. Upward
currents of air, whirlwinds and tornadoes, occur in all parts
of the world, and large numbers of insects are thus carried
up into the higher regions of the atmosphere, where they
are liable to be caught by strong winds, and thus conveyed
enormous distances over seas or continents. With such

1 Trans. Ent. Soc, 1871, p. 1S4.

xii GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 361

powerful means of dispersal the distribution of insects over
the entire globe, and their presence in the most remote
oceanic islands, offer no difficulties.

The Dispersal of Plants.

The dispersal of seeds is effected in a greater variety of
ways than are available in the case of any animals. Some
fruits or seed-vessels, and some seeds, will float for many
weeks, and after immersion in salt water for that period
the seeds will often germinate. Extreme cases are the double
cocoa-nut of the Seychelles, which has been found on the coast
of Sumatra, about 3000 miles distant; the fruits of the
Sapindus saponaria (soap-berry), which has been brought to
Bermuda by the Gulf Stream from the West Indies, and has
grown after a journey in the sea of about 1500 miles ; and the
West Indian bean, Entada scandens, which reached the Azores
from the West Indies, a distance of full 3000 miles, and after-
wards germinated at Kew. By these means we can account
for the similarity in the shore flora of the Malay Archipelago
and most of the islands of the Pacific ; and from an examination
of the fruits and seeds, collected among drift during the voyage
of the Challenger, Mr. Hemsley has compiled a list of 121
species which are probably widely dispersed by this means.

A still larger number of species owe their dispersal to birds
in several distinct ways. An immense number of fruits in all
parts of the world are devoured by birds, and have been
attractively coloured (as we have seen), in order to be so
devoured, because the seeds pass through the birds' bodies and
germinate where they fall. We have seen how frequently
birds are forced by gales of wind across a wide expanse of
ocean, and thus seeds must be occasionally carried. It is a
very suggestive fact, that all the trees and shrubs in the Azores
bear berries or small fruits which are eaten by birds ; while all
those which bear larger fruits, or are eaten chiefly by mammals
— such as oaks, beeches, hazels, crabs, etc. — are entirely
wanting. Game-birds and waders often have portions of mud
attached to their feet, and Mr. Darwin has proved by experi-
ment that such mud frequently contains seeds. One partridge
had such a quantity of mud attached to its foot as to contain
seeds from which eighty -two plants germinated; this proves that

362 DARWINISM

a very small portion of mud may serve to convey seeds, and
such an occurrence repeated even at long intervals may greatly
aid in stocking remote islands with vegetation. Many seeds
also adhere to the feathers of birds, and thus, again, may be
conveyed as far as birds are ever carried. Dr. Guppy found
a small hard seed in the gizzard of a Cape Petrel, taken about
550 miles east of Tristan da Cunha.

Dispersal of Seeds by the Wind.

In the preceding cases we have been able to obtain direct
evidence of transportal; but although we know that many seeds
are specially adapted to be dispersed by the wind, we cannot
obtain direct proof that they are so carried for hundreds or
thousands of miles across the sea, owing to the difficulty of
detecting single objects which are so small and inconspicuous.
It is probable, however, that the wind as an agent of dispersal
is really more effective than any of those we have hitherto
considered, because a very large number of plants have seeds
which are very small and light, and are often of such a form
as to facilitate aerial carriage for enormous distances. It is
evident that such seeds are especially liable to be transported
by violent winds, because they become ripe in autumn at the
time when storms are most prevalent, while they either lie
upon the surface of the ground, or are disposed in dry capsules
on the plant ready to be blown away. If inorganic particles
comparable in weight, size, or form with such seeds are
carried for great distances, we may be sure that seeds will also
be occasionally carried in the same way. It will, therefore,
be necessary to give a few examples of wind-carriage of small
objects.

On 27th July 1875 a remarkable shower of small pieces of
hay occurred at Monkstown, near Dublin. They appeared
floating slowly down from a great height, as if falling from a
dark cloud which hung overhead. The pieces picked up were
wet, and varied from single blades of grass to tufts weighing
one or two ounces. A similar shower occurred a few days
earlier in Denbighshire, and was observed to travel in a
direction contrary to that of the wind in the lower atmosphere.1
There is no evidence of the distance from which the hay was
1 Nature (1875), vol. xii. pp. 279, 298.

xii GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 363

brought, but as it had been carried to a great height, it was
in a position to be conveyed to almost any distance by a
violent wind, had such occurred at the time.

Mineral Matter carried by the Wind.

The numerous cases of sand and volcanic dust being carried
enormous distances through the atmosphere sufficiently prove
the importance of wind as a carrier of solid matter, but un-
fortunately the matter collected has not been hitherto examined
with a view to determine the maximum size and weight of the
particles. A few facts, however, have been kindly furnished
me by Professor Judd, F.R.S. Some dust which fell at
Genoa on 15th October 1885, and was believed to have been
brought from the African desert, consisted of quartz, horn-
blende, and other minerals, and contained particles having a
diameter of -g-thr mc^' eac^ weighing 200W0 grain. This
dust had probably travelled over 600 miles. In the dust from
Krakatoa, which fell at Batavia, about 100 miles distant,
during the great eruption, there are many solid particles even
larger than those mentioned above. Some of this dust was given
me by Professor Judd, and I found in it several ovoid particles
of a much larger size, being -£$ inch long, and TV wide and
deep. The dust from the same eruption, which fell on board
the ship Arabella, 970 miles from the volcano, also contained
solid particles T^ inch diameter. Mr. John Murray of the
Challenger Expedition writes to me that he finds in the deep
sea deposits 500 and even 700 miles west of the coast of
Africa, rounded particles of quartz, having a diameter of
•^-i^ inch, and similar particles are found at equally great
distances from the south-west coasts of Australia ; and he
considers these to be atmospheric dust carried to that
distance by the wind. Taking the sp. gr. of quartz at
2-6, these particles would weigh about -g-syihnj- grain each.
These interesting facts can, however, by no means be taken
as indicating the extreme limits of the power of wind in
carrying solid particles. During the Krakatoa eruption
no gale of special violence occurred, and the region is
one of comparative calms. The grains of quartz found by
Mr. Murray more nearly indicate the limit, but the very
small portions of matter brought up by the dredge, as com-

364

DARWINISM

pared with the enormous areas of sea-bottom, over which the
atmospheric dust must have been scattered, render it in the
highest degree improbable that the maximum limit either of
size of particles, or of distance from land has been reached.

Let us, however,, assume that the quartz grains, found by-
Mr. Murray in the deep-sea ooze 700 miles from land, give us
the extreme limit of the power of the atmosphere as a carrier
of solid particles, and let us compare with these the weights
of some seeds. From a small collection of the seeds of thirty

No.

Species.

Approximate
No. of Seeds
in one Grain.

Approximate
Dimensions.

Kemarks.

in. in. in.

1

Draba verna

1,800

1 Y 1 y 1

TO" x "5T x T6~0

Oval, flat.

2

Hypericum perforatum

520

1 v 1

-arc x"ro

Cylindrical.

3

Astilbe rivularis .

4,500

i v i

TO" x T0"0"

Elongate, flat, tailed, wavy.

4

Saxifraga coriophylla .

750

1 v 1

Surface rough, adhere to the
dry capsules.

5

(Enothera rosea .

640

1 v, 1

TTT X¥TT

Ovate.

6

Hypericum hirsutum .

700

1 v 1

7tr x tuh

Cylindrical, rough.

7

Mimulus luteus .

2,900

1 v 1

7TTJ" x TUTS

Oval, minute.

8

Penthorum sedoides .

8,000*

1 v 1
7"0~XT"3TT

Flattened, very minute.

9

Sagina procumbens

12,000*

1

TTU

Sub-triangular, flat.

10

Orchis maculata

15,000*

Margined, flat, very minute.

11

Gentiana purpurea

35

1

Wavy, rough, with this cori-
aceous margins.

12

Silene alpina

1
TTS

Flat, with fringed margins.

13

Adenopliora communis

1 Y 1

T0"xT0"

Very thin, wavy, light.

Quartz grains

25,000

1

Deep sea . . 700 miles.

Do.

200,000

rUTT

Genoa . . . 600 miles.

species of herbaceous plants sent me from Kew, those in the
above table were selected, and small portions of eight of
them carefully weighed in a chemical balance.1 By counting
these portions I was able to estimate the number of seeds
weighing one grain. The three very minute species, whose
numbers are marked with an asterisk (*), were estimated by
the comparison of their sizes with those of the smaller weighed
seeds.

If now we compare the seeds with the quartz grains, we

1 I am indebted to Professor R. Meldola of the Finsbury Technical Institute,
and Rev. T. D. Titmas of Charterhouse for furnishing me with the weights
required.

xi r GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 365

find that several are from twice to three times the weight
of the grains found by Mr. Murray, and others five times,
eight times, and fifteen times as heavy ; but they are pro-
portionately very much larger, and, being usually irregular in
shape or compressed, they expose a very much larger surface to
the air. The surface is often rough, and several have dilated
margins or tailed appendages, increasing friction and rendering
the uniform rate of falling through still air immensely less
than in the case of the smooth, rounded, solid quartz grains.
With these advantages it is a moderate estimate that seeds
ten times the weight of the quartz grains could be carried
quite as far through the air by a violent gale and under the
most favourable conditions. These limits will include five
of the seeds here given, as well as hundreds of others which do
not exceed them in weight ; and to these we may add some
larger seeds which have other favourable characteristics, as is
the case with numbers 11-13, which, though very much larger
than the rest, are so formed as in all probability to be still more
easily carried great distances by a gale of wind. It appears,
therefore, to be absolutely certain that every autumnal gale
capable of conveying solid mineral particles to great distances,
must also carry numbers of small seeds at least as far ; and if
this is so, the wind alone will form one of the most effective
agents in the dispersal of plants.

Hitherto this mode of conveyance, as applying to the
transmission of seeds for great distances across the ocean, has
been rejected by botanists, for two reasons. In the first place,
there is said to be no direct evidence of such conveyance ; and,
secondly, the peculiar plants of remote oceanic islands do not
appear to have seeds specially adapted for aerial transmission.
I will consider briefly each of these objections.

Objection to the Theory of Wind- Dispersal.

To obtain direct evidence of the transmission of such
minute and perishable objects, which do not exist in great
quantities, and are probably carried to the greatest distances
but rarely and as single specimens, is extremely difficult. A
bird or insect can be seen if it comes on board ship, but who
would ever detect the seeds of Mimulus or Orchis even if a
score of them fell on a ship's deck 1 Yet if but one such seed

366 DARWINISM chap.

per century were carried to an oceanic island, that island
might become rapidly overrun by the plant, if the conditions
were favourable to its growth and reproduction. It is further
objected that search has been made for such seeds, and they
have not been found. Professor Kerner of Innsbruck examined
the snow on the surface of glaciers, and assiduously collected
all the seeds he could find, and these were all of plants which
grew in the adjacent mountains or in the same district. In
like manner, the plants growing on moraines were found to
be those of the adjacent mountains, plateaux, or lowlands.
Hence he concluded that the prevalent opinion that seeds
may be carried through the air for very great distances " is
not supported by fact." 1 The opinion is certainly not
supported by Kerner's facts, but neither is it opposed by
them. It is obvious that the seeds that would be carried by
the wind to moraines or to the surface of glaciers would be, first
and in the greatest abundance, those of the immediately
surrounding district; then, very much more rarely, those
from more remote mountains ; and lastly, in extreme rarity,
those from distant countries or altogether distinct mountain
ranges. Let us suppose the first to be so abundant that a
single seed could be found by industrious search on each
square yard of the surface of the glacier ; the second so scarce
that only one could possibly be found in a hundred yards
square ; while to find one of the third class it would be
necessary exhaustively to examine a square mile of surface.
Should we expect that one ever to be found, and should the fact
that it could not be found be taken as a proof that it was not
there ? Besides, a glacier is altogether in a bad position to
receive such remote A^anderers, since it is generally surrounded
by lofty mountains, often range behind range, which would
intercept the few air-borne seeds that might have been carried
from a distant land. The conditions in an oceanic island, on the
other hand, are the most favourable, since the land, especially
if high, will intercept objects carried by the wind, and will
thus cause more of the solid matter to fall on it than on an
equal area of ocean. We know that winds at sea often blow
violently for days together, and the rate of motion is indicated
by the fact that 72 miles an hour was the average velocity
1 See Nature, vol. vi. p. 164, for a summary of Kerner's paper.

xii GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 307

of the wind observed during twelve hours at the Ben Nevis
observatory, while the velocity sometimes rises to 120 miles
an hour. A twelve hours' gale might, therefore, carry
light seeds a thousand miles as easily and certainly as it
could carry quartz -grains of much greater specific gravity,
rotundity, and smoothness, 500 or even 100 miles; and it is
difficult even to imagine a sufficient reason why they should
not be so carried — perhaps very rarely and under exceptionally
favourable conditions, — but this is all that is required.

As regards the second objection, it has been observed that
orchidese, which have often exceedingly small and light seeds,
are remarkably absent from oceanic islands. This, however,
may be very largely due to their extreme specialisation and
dependence on insect agency for their fertilisation ; while the
fact that they do occur in such very remote islands as the
Azores, Tahiti, and the Sandwich Islands, proves that they
must have once reached these localities either by the agency
of birds or by transmission through the air ; and the facts I
have given above render the latter mode at least as probable
as the former. Sir Joseph Hooker remarks on the composite
plant of Kerguelen Island (Cotula plumosa) being found also on
Lord Auckland and MacQuarrie Islands, and yet having no
pappus, while other species of the genus possess it. This is
certainly remarkable, and proves that the plant must have, or
once have had, some other means of dispersal across wide
oceans.1 One of the most widely dispersed species in the
whole world (Sonchus oleraceus) possesses pappus, as do four
out of five of the species which are common to Europe and
New Zealand, all of which have a very wide distribution.
The same author remarks on the limited area occupied by
most species of Compositae, notwithstanding their facilities for
dispersal by means of their feathered seeds ; but it has been

1 It seems quite possible that the absence of pappus in this case is a recent
adaptation, and that it has been brought about by causes similar to those
which have reduced or aborted the wings of insects in oceanic islands. For
when a plant has once reached one of the storm-swept islands of the southern
ocean, the pappus will be injurious for the same reason that the wings of
insects are injurious, since it will lead to the seeds being blown out to sea and
destroyed. The seeds which are heaviest and have least pappus will have the
best chance of falling on the ground and remaining there to germinate, and
this process of selection might rapidly lead to the entire disappearance of the
pappus.

368 DARWINISM

already shown that limitations of area are almost always due
to the competition of allied forms, facilities for dispersal being
only one of many factors in determining the wide range of
species. It is, however, a specially important factor in the
case of the inhabitants of remote oceanic islands, since, whether
they are peculiar species or not, they or their remote ancestors
must at some time or other have reached their present posi-
tion by natural means.

I have already shown elsewhere, that the flora of the
Azores strikingly supports the view of the species having been
introduced by aerial transmission only, that is, by the agency
of birds and the wind, because all plants that could not possibly
have been carried by these means are absent.1 In the same
way we may account for the extreme rarity of Leguminosse in
all oceanic islands. Mr. Hemsley, in his Report on Insular
Floras, says that they "are wanting in a large number of
oceanic islands where there is no true littoral flora," as St.
Helena, Juan Fernandez, and all the islands of the South
Atlantic and South Indian Oceans. Even in the tropical
islands, such as Mauritius and Bourbon, there are no endemic
species, and very few in the Galapagos and the remoter Pacific
Islands. All these facts are quite in accordance with the absence
of facilities for transmission through the air, either by birds
or the wind, owing to the comparatively large size and weight
of the seeds ; and an additional proof is thus afforded of the
extreme rarity of the successful floating of seeds for great
distances across the ocean.2

Explanation of North Temperate Plants in the Southern Hemisphere.

If we now admit that many seeds which are either minute
in size, of thin texture or wavy form, or so fringed or
margined as to afford a good hold to the air, are capable of
being carried for many hundreds of miles by exceptionally

1 See Island Life, p. 251.

2 Mr. Hemsley suggests that it is not so much the difficulty of transmission
by floating, as the bad conditions the seeds are usually exposed to when they
reach land. Many, even if they germinate, are destroyed by the waves, as
Burchell noticed at St. Helena ; while even a flat and sheltered shore would
be an unsuitable position for many inland plants. Air-borne seeds, on the
other hand, may be carried far inland, and so scattered that some of them
are likely to reach suitable stations.

xii GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 369

violent and long-continued gales of wind, we shall not only be
better able to account for the floras of some of the remotest
oceanic islands, but shall also find in the fact a sufficient ex-
planation of the wide diffusion of many genera, and even species,
of arctic and north temperate plants in the southern hemisphere
or on the summits of tropical mountains. Nearly fifty of the
flowering plants of Tierra-del-Fuego are found also in North
America or Europe, but in no intermediate country ; while fifty-
eight species are common to New Zealand and Northern Europe ;
thirty-eight to Australia, Northern Europe, and Asia ; and no
less than seventy-seven common to New Zealand, Australia,
and South America.1 On lofty mountains far removed from each
other, identical or closely allied plants often occur. Thus the
fine Primula imperialis of a single mountain peak in Java has
been found (or a closely allied species) in the Himalayas ;
and many other plants of the high mountains of Java, Ceylon,
and North India are either identical or closely allied forms. So,
in Africa, some species, found on the summits of the Cameroons
and Fernando Po in West Africa, are closely allied to species
in the Abyssinian highlands and in Temperate Europe ; while
other Abyssinian and Cameroons species have recently been
found on the mountains of Madagascar. Some peculiar Aus-
tralian forms have been found represented on the summit of
Kini Balu in Borneo. Again, on the summit of the Organ
mountains in Brazil there are species allied to those of the
Andes, but not found in the intervening lowlands.

No Proof of Recent Lower Temperature in the Tropics.

Now all these facts, and numerous others of like character,
were supposed by Mr. Darwin to be due to a lowering of
temperature during glacial epochs, which allowed these tem-
perate forms to migrate across the intervening tropical low-
lands. But any such change within the epoch of existing species
is almost inconceivable. In the first place, it would necessitate
the extinction of much of the tropical flora (and with it of the
insect life), because without such extinction alpine herbaceous
plants could certainly never spread over tropical forest low-

1 For fuller particulars, see Sir J. Hooker's Introduction to Floras of New
Zealand and Australia, and a summary in my Island Life, chaps, xxii.
xxiii.

2 B

370 DARWINISM chap.

lands ; and, in the next place, there is not a particle of direct
evidence that any such lowering of temperature in inter-
tropical lowlands ever took place. The only alleged evidence
of the kind is that adduced by the late Professor Agassiz and
Mr. Hartt ; but L am informed by my friend, Mr. J. C. Branner
(now State Geologist of Arkansas, U.S.), who succeeded Mr.
Hartt, and spent several years completing the geological
survey of Brazil, that the supposed moraines and glaciated
granite rocks near Bio Janeiro and elsewhere, as well as the
so-called boulder -clay of the same region, are entirely ex-
plicable as the results of sub-aerial denudation and weathering,
and that there is no proof whatever of glaciation in any
part of Brazil.

Lower Temperature not needed to Explain the Facts.

But any such vast physical change as that suggested by
Darwin, involving as it does such tremendous issues as re-
gards its effects on the tropical fauna and flora of the whole
world, is really quite uncalled for, because the facts to be
explained are of the same essential nature as those presented
by remote oceanic islands, between which and the nearest con-
tinents no temperate land connection is postulated. In pro-
portion to their limited area and extreme isolation, the Azores,
St. Helena, the Galapagos, and the Sandwich Islands, each
possess a fairly rich — the last a very rich — indigenous flora ;
and the means which sufficed to. stock them with a great
variety of plants would probably suffice to transmit others
from mountain-top to mountain-top in various parts of the
globe. In the case of the Azores, we have large numbers of
species identical with those of Europe, and others closely allied,
forming an exactly parallel case to the species found on the
various mountain summits which have been referred to. The
distances from Madagascar to the South African mountains
and to Kilimandjaro, and from the latter to Abyssinia, are no
greater than from Spain to the Azores, while there are other
equatorial mountains forming stepping-stones at about an
equal distance to the Cameroons. Between Java and the
Himalayas we have the lofty mountains of Sumatra and of
North-western Burma, forming steps at about the same distance
apart ; while between Kini Balu and the Australian Alps we

xa GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 371

have the unexplored snow mountains of New Guinea, the
Bellenden Ker mountains in Queensland, and the New England
and Blue Mountains of New South Wales. Between Brazil
and Bolivia the distances are no greater ; while the unbroken
range of mountains from Arctic America to Tierra-del-Fuego
offers the greatest facilities for transmission, the partial gap
between the lofty peak of Chiriqui and the high Andes of New
Grenada being far less than from Spain to the Azores. Thus,
whatever means have sufficed for stocking oceanic islands must
have been to some extent effective in transmitting northern
forms from mountain to mountain, across the equator, to the
southern hemisphere ; while for this latter form of dispersal
there are special facilities, in the abundance of fresh and un-
occupied surfaces always occurring in mountain regions, owing
to avalanches, torrents, mountain-slides, and rock-falls, thus
affording stations on which air-borne seeds may germinate
and find a temporary home till driven out by the inroads of
the indigenous vegetation. These temporary stations may be
at much lower altitudes than the original habitat of the species,
if other conditions are favourable. Alpine plants often descend
into the valleys on glacial moraines, while some arctic species
grow equally well on mountain summits and on the seashore.
The distances above referred to between the loftier mountains
may thus be greatly reduced by the occurrence of suitable
conditions at lower altitudes, and the facilities for trans-
mission by means of aerial currents proportionally increased.1

Facts Explained by the Wind-Carriage of Seeds.

But if we altogether reject aerial transmission of seeds for
great distances, except by the agency of birds, it will be
difficult, if not impossible, to account for the presence of so
many identical species of plants on remote mountain summits,
or for that " continuous current of vegetation " described by
Sir Joseph Hooker as having apparently long existed from
the northern to the southern hemisphere. It may be admitted
that we can, possibly, account for the greater portion of the
floras of remote oceanic islands by the agency of birds alone ;
because, when blown out to sea land-birds must reach some island

1 For a fuller discussion of this subject, see my Island Life, chap, xxiii.

372 DARWINISM

or perish, and all which come within sight of an island will
struggle to reach it as their only refuge. But, with mountain
summits the case is altogether different, because, being sur-
rounded by land instead of by sea, no bird would need to fly,
or to be carried by the wind, for several hundred miles at a
stretch to another mountain summit, but would find a refuge
in the surrounding uplands, ridges, valleys, or plains. As a
rule the birds that frequent lofty mountain tops are peculiar
species, allied to those of the surrounding district ; and there
is no indication whatever of the passage of birds from one
remote mountain to another in any way comparable with
the flights of birds which are known to reach the Azores
annually, or even with the few regular migrants from
Australia to New Zealand. It is almost impossible to con-
ceive that the seeds of the Himalayan primula should have
been thus carried to Java ; but, by means of gales of wind,
and intermediate stations from fifty to a few hundred miles
apart, where the seeds might vegetate for a year or two and
produce fresh seed to be again carried on in the same
manner, the transmission might, after many failures, be at
last effected.

A very important consideration is the vastly larger scale
on which wind-carriage of seeds must act, as compared with
bird-carriage. It can only be a few birds which carry seeds
attached to their feathers or feet. A very small proportion of
these would carry the seeds of Alpine plants ; while an almost
infinitesimal fraction of these latter would convey the few
seeds attached to them safely to an oceanic island or remote
mountain. But winds, in the form of Avhirlwincls or tornadoes,
gales or hurricanes, are perpetually at work over large areas
of land and sea. Insects and light particles of matter are
often carried up to the tops of high mountains ; and, from the
very nature and origin of winds, they usually consist of
ascending or descending currents, the former capable of
suspending such small and light objects as are many seeds
long enough for them to be carried enormous distances. For
each single seed carried away by external attachment to the
feet or feathers of a bird, countless millions are probably
carried away by violent winds ; and the chance of conveyance
to a great distance and in a definite direction must be many

xn GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 373

times greater by the latter mode than by the former.1 We
have seen that inorganic particles of much greater specific
gravity than seeds, and nearly as heavy as the smallest kinds,
are carried to great distances through the air, and we can
therefore hardly doubt that some seeds are carried as far.
The direct agency of the wind, as a supplement to bird-
transport, will help to explain the presence in oceanic islands
of plants growing in dry or rocky places whose small seeds
are not likely to become attached to birds ; while it seems to
be the only effective agency possible in the dispersal of those
species of alpine or sub-alpine plants found on the summits
of distant mountains, or still more widely separated in the
temperate zones of the northern and southern hemispheres.

Concluding Remarks.

On the general principles that have been now laid down, it
will be found that all the chief facts of the geographical dis-
tribution of animals and plants can be sufficiently understood.
There will, of course, be many cases of difficulty and some
seeming anomalies, but these can usually be seen to depend on
our ignorance of some of the essential factors of the problem.
Either we do not know the distribution of the group in recent
geological times, or we are still ignorant of the special methods
by which the organisms are able to cross the sea. The latter
difficulty applies especially to the lizard tribe, which are found

1 A very remarkable case of wind conveyance of seeds on a large scale is
described in a letter from Mr. Thomas Hanbury to his brother, the late
Daniel Hanbury, which has been kindly communicated to me by Mr. Hemsley
of Kew. The letter is dated "Shanghai, 1st May 1856," and the passage
referred to is as follows : —

" For the past three days we have had very warm weather for this time of
year, in fact almost as warm as the middle of summer. Last evening the
wind suddenly changed round to the north and blew all night with consider-
able violence, making a great change in the atmosphere.

' ' This morning, myriads of small white particles are floating about in the
air ; there is not a single cloud and no mist, yet the sun is quite obscured by
this substance, and it looks like a white fog in England. I enclose thee a
sample, thinking it may interest. It is evidently a vegetable production ; I
think, apparently, some kind of seed. "

Mr. Hemsley adds, that this substance proves to be the plumose seeds of
a poplar or willow. In order to produce the effects described — quite obscuring
the sun like a white fog, — the seeds must have filled the air to a very great
height ; and they must have been brought from some district where there were
extensive tracts covered with the tree which produced them.

374 DARWINISM

in almost all the tropical oceanic islands ; but the particular
mode in which they are able to traverse a wide expanse of
ocean, which is a perfect barrier to batrachia and almost so to
snakes, has not yet been discovered. Lizards are found in all
the larger Pacific. Islands as far as Tahiti, while snakes do not
extend beyond the Fiji Islands ; and the latter are also absent
from Mauritius and Bourbon, where lizards of seven or eight
species abound. Naturalists resident in the Pacific Islands
would make a valuable contribution to our science by study-
ing the life-history of the native lizards, and endeavouring to
ascertain the special facilities they possess for crossing over
wide spaces of ocean.

CHAPTER XIII

THE GEOLOGICAL EVIDENCES OF EVOLUTION

What we may expect — The number of known species of extinct animals —
Causes of the imperfection of the geological record — Geological
evidences of evolution — Shells — -Crocodiles — The rhinoceros tribe —
The pedigree of the horse tribe — Development of deer's horns — Brain
development — Local relations of fossil and living animals — Cause of
extinction of laige animals — Indications of general progress in plants
and animals — The progressive development of plants — Possible cause
of sudden late appearance of exogens — Geological distribution of
insects — Geological succession of vertebrata — Concluding remarks.

The theory of evolution in the organic world necessarily im-
plies that the forms of animals and plants have, broadly
speaking, progressed from a more generalised to a more
specialised structure, and from simpler to more complex
forms. We know, however, that this progression has been
by no means regular, but has been accompanied by repeated
degradation and degeneration ; while extinction on an
enormous scale has again and again stopped all progress in
certain directions, and has often compelled a fresh start
in development from some comparatively low and imperfect
type.

The enormous extension of geological research in recent
times has made us acquainted with a vast number of extinct
organisms, so vast that in some important groups— such as
the mollusca — the fossil are more numerous than the living
species ; while in the mammalia they are not much less
numerous, the preponderance of living species being chiefly in
the smaller and in the arboreal forms which have not been so
well preserved as the members of the larger groups. With
such a wealth of material to illustrate the successive stages

376 DARWINISM

through which animals have passed, it will naturally be ex-
pected that we should find important evidence of evolution.
We should hope to learn the steps by which some isolated
forms have been connected with their nearest allies, and in
many cases to have the gaps filled up which now separate
genus from genus,*or species from species. In some cases these
expectations are fulfilled, but in many other cases we seek
in vain for evidence of the kind we desire ; and this absence
of evidence with such an apparent wealth of material is held
by many persons to throw doubt on the theory of evolution
itself. They urge, with much appearance of reason, that all
the arguments we have hitherto adduced fall short of demon-
stration, and that the crucial test consists in being able to
show, in a great number of cases, those connecting links which
we say must have existed. Many of the gaps that still remain
are so vast that it seems incredible to these writers that they
could ever have been filled up by a close succession of species,
since these must have spread over so many ages, and have
existed in such numbers, that it seems impossible to account
for their total absence from deposits in which great numbers
of species belonging to other groups are preserved and have
been discovered. In order to appreciate the force, or weakness,
of these objections, Ave must inquire into the character and
completeness of that record of the past life of the earth which
geology has unfolded, and ascertain the nature and amount
of the evidence which, under actual conditions, we may expect
to find.

The Number of known Species of Extinct Animals.

When we state that the known fossil mollusca are consider-
ably more numerous than those which now live on the earth,
it appears at first sight that our knowledge is very complete,
but this is far from being the case. The species have been
continually changing throughout geological time, and at each
period have probably been as numerous as they are now. If
we divide the fossiliferous strata into twelve great divisions
— the Pliocene, Miocene, Eocene, Cretaceous, Oolite, Lias,
Trias, Permian, Carboniferous, Devonian, Silurian, and Cam-
brian,—we find not only that each has a very distinct and
characteristic molluscan fauna, but that the different sub-

xin THE GEOLOGICAL EVIDENCES OF EVOLUTION 377

divisions often present a widely different series of species ; so
that although a certain number of species are common to
two or more of the great divisions, the totality of the species
that have lived upon the earth must be very much more than
twelve times — perhaps even thirty or forty times — the
number now living. In like manner, although the species of
fossil mammals now recognised by more or less fragmentary
fossil remains may not be much less numerous than the
living species, yet the duration of existence of these was
comparatively so short that they were almost completely
changed, perhaps six or seven times, during the Tertiary
period ; and this is certainly only a fragment of the geological
time during which mammalia existed on the globe.

There is also reason to believe that the higher animals
were much more abundant in species during past geological
epochs than now, owing to the greater equability of the climate
which rendered even the arctic regions as habitable as the
temperate zones are in our time.

The same equable climate would probably cause a more
uniform distribution of moisture, and render what are now
desert regions capable of supporting abundance of animal life.
This is indicated by the number and variety of the species of
large animals that have been found fossil in very limited areas
which they evidently inhabited at one period. M. Albert
Gaudry found, in the deposits of a mountain stream at
Pikermi in Greece, an abundance of large mammalia such as
are nowhere to be found living together at the present time.
Among them were two species of Mastodon, two different
rhinoceroses, a gigantic wild boar, a camel and a giraffe
larger than those now living, several monkeys, carnivora
ranging from martens and civets . to lions and hyaenas of the
largest size, numerous antelopes of at least five distinct genera,
and besides these many forms altogether extinct. Such were
the great herds of Hipparion, an ancestral form of horse ; the
Helladotherium, a huge animal bigger than the giraffe ; the
Ancylotherium, one of the Edentata ; the huge Dinotherium ;
the Aceratherium, allied to the rhinoceros ; and the monstrous
Chalicotherium, allied to the swine and ruminants, but as large
as a rhinoceros ; and to prey upon these, the great Mac-
hairodus or sabre-toothed tisrer. And all these remains were

378 DARWINISM

found in a space 300 paces long by 60 paces broad, many of
the species existing in enormous quantities.

The Pikermi fossils belong to the Upper Miocene forma-
tion, but an equally rich deposit of Upper Eocene age has
been discovered in South- Western France at Quercy, where M.
Filhol has determined the presence of no less than forty-two
species of beasts of prey alone. Equally remarkable are the
various discoveries of mammalian fossils in North America,
especially in the old lake bottoms now forming what are
called the " bad lands " of Dakota and Nebraska, belonging to
the Miocene period. Here are found an enormous assemblage
of remains, often perfect skeletons, of herbivora and carnivora,
as varied and interesting as those from the localities already
referred to in Europe ; but altogether distinct, and far ex-
ceeding, in number and variety of species of the larger animals,
the whole existing fauna of North America. Very similar
phenomena occur in South America and in Australia, leading
us to the conclusion that the earth at the present time is
impoverished as regards the larger animals, and that at each
successive period of Tertiary time, at all events, it contained
a far greater number of species than now inhabit it. The
very richness and abundance of the remains which Ave find
in limited areas, serve to convince us how imperfect and
fragmentary must be our knowledge of the earth's fauna at
any one past epoch ; since we cannot believe that all, or
nearly all, of the animals which inhabited any district were
entombed in a single lake, or overwhelmed by the floods of a
single river.

But the spots where such rich deposits .occur are ex-
ceedingly few and far between when compared with the vast
areas of continental land, and we have every reason to believe
that in past ages, as now, numbers of curious species were
rare or local, the commoner and more abundant species giving
a very imperfect idea of the existing series of animal forms.
Yet more important, as showing the imperfection of our
knowledge, is the enormous lapse of time between the several
formations in which we find organic remains in any abundance,
so vast that in many cases we find ourselves almost in a new
world, all the species and most of the genera of the higher
animals having undergone a complete change.

THE GEOLOGICAL EVIDENCES OF EVOLUTION 379

Causes of the Imperfection of the Geological Record.

These facts are quite in accordance with, the conclusions of
geologists as to the necessary imperfection of the geological
record, since it requires the concurrence of a number of
favourable conditions to preserve any adequate representation
of the life of a given epoch. In the first place, the animals to
be preserved must not die a natural death by disease, or old
age, or by being the prey of other animals, but must be
destroyed by some accident which shall lead to their being-
embedded in the soil. They must be either carried away by
floods, sink into bogs or quicksands, or be enveloped in the
mud or ashes of a volcanic eruption ; and when thus embedded
they must remain undisturbed amid all the future changes of
the earth's surface.

But -the chances against this are enormous, because de-
nudation is always going on, and the rocks we now find at
the earth's surface are only a small fragment of those which
were originally laid down. The alternations of marine and
freshwater deposits, and the frequent unconformability of
strata with those which overlie them, tell us plainly of
repeated elevations and depressions of the surface, and of
denudation on an enormous scale. Almost every mountain
range, with its peaks, ridges, and valleys, is but the remnant
of some vast plateau eaten away by sub-aerial agencies ; every
range of sea-cliffs tell us of long slopes of land destroyed by
the waves ; while almost all the older rocks which now form
the surface of the earth have been once covered with newer
deposits which have long since disappeared. Nowhere are
the evidences of this denudation more apparent than in North
and South America, where granitic or metamorphic rocks cover
an area hardly less than that of all Europe. The same rocks
are largely developed in Central Africa and Eastern Asia ;
while, besides those portions that appear exposed on the
surface, areas of unknown extent are buried under strata
which rest on them uncomformably, and could not, there-
fore, constitute the original capping under which the whole of
these rocks must once have been deeply buried ; because
granite can only be formed, and metamorphism can only go
on, deep clown in the crust of the earth. What an over-

380 DARWINISM

whelming idea does this give us of the destruction of whole
piles of rock, miles in thickness and covering areas comparable
with those of continents ; and how great must have been the
loss of the innumerable fossil forms which those rocks con-
tained ! In view of such destruction we are forced to conclude
that our palseontological collections, rich though they may-
appear, are really but small and random samples, giving no
adequate idea of the mighty series of organism which have
lived upon the earth.1

Admitting, however, the extreme imperfection of the geo-
logical record as a whole, it may be urged that certain limited
portions of it are fairly complete — as, for example, the various
Miocene deposits of India, Europe, and North America,- — ■
and that in these we ought to find many examples of species
and genera linked together by intermediate forms. It may be
replied that in several cases this really occurs ; and the reason
why it does not occur more often is, that the theory of
evolution requires that distinct genera should be linked
together, not by a direct passage, but by the descent of both
from a common ancestor, which may have lived in some much
earlier age the record of which is either wanting or very in-
complete. An illustration given by Mr. Darwin will make this
more clear to those who have not studied the subject. The
fantail and pouter pigeons are two very distinct and unlike
breeds, which we yet know to have been both derived from the
common wild rock-pigeon. Now, if we had every variety of
living pigeon before us, or even all those which have lived
during the present century, we should find no intermediate
types between these two — none combining in any degree the
characters of the pouter with that of the fantail. Neither
should we ever find such an intermediate form, even had there
been preserved a specimen of every breed of pigeon since
the ancestral rock -pigeon was first tamed by man — a
period of probably several thousand years. We thus see
that a complete passage from one very distinct species to
another could not be expected even had we a complete record
of the life of any one period. What Ave require is a complete

1 The reader who desires to understand this subject more fully, should
study chap. x. of the Origin of Species, and chap. xiv. of Sir Charles Lyell's
Principles of Geology.

xin THE GEOLOGICAL EVIDENCES OF EVOLUTION 381

record of all the species that have existed since the two forms
began to diverge from their common ancestor, and this the
known imperfection of the record renders it almost impossible
that we should ever attain. All that we have a right to
expect is, that, as we multiply the fossil forms in any group,
the gaps that at first existed in that group shall become less
wide and less numerous ; and also that, in some cases, a tolerably
direct series shall be found, by which the more specialised
forms of the present day shall be connected with more
generalised ancestral types. We might also expect that when
a country is now characterised by special groups of animals,
the fossil forms that immediately preceded them shall, for the
most part, belong to the same groups ; and further, that, com-
paring the more ancient with the more modern types, we
should find indications of progression, the earlier forms being,
on the -whole, lower in organisation, and less specialised in
structure than the later. Now evidence of evolution of these
varied kinds is what we do find, and almost every fresh discovery
adds to their number and cogency. In order, therefore, to show
that the testimony given by geology is entirely in favour of
the theory of descent with modification, some of the more
striking of the facts will now be given.

Geological Evidences of Evolution.

In an article in Nature (vol. xiv. p. 275), Professor Jucld
calls attention to some recent discoveries in the Hungarian
plains, of fossil lacustrine shells, and their careful study by Dr.
Neumayr and M. Paul of the Austrian Geological Survey.
The beds in which they occur have accumulated to the thick-
ness of 2000 feet, containing throughout abundance of fossils,
and divisible into eight zones, each of which exhibits a well-
marked and characteristic fauna. Professor Judcl then de-
scribes the bearing of these discoveries as follows —

" The group of shells which affords the most interesting
evidence of the origin of new forms through descent with modi-
fication is that of the genus Vivipara or Paludina, which occurs
in prodigious abundance throughout the whole series of fresh-
water strata. We shall not, of course, attempt in this place
to enter into any details concerning the forty distinct forms of
this genus (Dr. Neumayr very properly hesitates to call them all

382 DARWINISM

species), which are named and described in this monograph, and
between which, as the authors show, so many connecting links,
clearly illustrating the derivation of the newer from the older
types, have been detected. On the minds of those who care-
fully examine the admirably engraved figures given in the
plates accompanying this valuable memoir, or still better, the
very large series of specimens from among which the subjects of
these figures are selected, and which are now in the museum
of the Eeichsanstalt of Vienna, but little doubt will, we
suspect, remain that the authors have fully made out their
case, and have demonstrated that, beyond all controversy, the
series with highly complicated ornamentation were variously
derived by descent — the lines of which are in most cases
perfectly clear and obvious — from the simple and unorna-
mented Vivipara achatinoides of the Congerien-Schichten (the
lower division of the series of strata). It is interesting to
notice that a large portion of these unquestionably derived
forms depart so widely from the type of the genus Vivipara,
that they have been separated on so high an authority as that
of Sandberger, as a new genus, under the name of Tulotoma.
And hence we are led to the conclusion that a vast number
of forms, certainly exhibiting specific distinctions, and accord-
ing to some naturalists, differences even entitled to be regarded
of generic value, have all a common ancestry."

It is, as Professor Judd remarks, owing to the exceptionally
favourable circumstances of a long -continued and unbroken
series of deposits being formed under physical conditions
either identical or very slowly changing, that we owe so com-
plete a record of the process of organic change. Usually,
some disturbing elements, such as a sudden change of physical
conditions, or the immigration of new sets of forms from other
areas and the consequent retreat or partial extinction of the
older fauna, interferes with the continuity of organic development,
and produces those puzzling discordances so generally met
with in geological formations of marine origin. While a case
of the kind now described affords evidence of the origin of
species complete and conclusive, though on a necessarily very
limited scale, the very rarity of the conditions which are essential
to such completeness serves to explain why it is that in most
cases the direct evidence of evolution is not to be obtained.

xiii THE GEOLOGICAL EVIDENCES OF EVOLUTION 383

Another illustration of the filling up of gaps between
existing groups is afforded by Professor Huxley's researches
on fossil crocodiles. The gap between the existing crocodiles
and the lizards is very wide, but as we go back in geological
time we meet with fossil forms which are to some extent
intermediate and form a connected series. The three living
genera — Crocodilus, Alligator, and Gavialis — are found in the
Eocene formation, and allied forms of another genus, Holops,
in the Chalk. From the Chalk backward to the Lias another
group of genera occurs, having anatomical characteristics
intermediate between the living crocodiles and the most
ancient forms. These, forming two genera Belodon and
Stagonolepis, are found in a, still older formation, the Trias.
They have characters resembling some lizards, especially the
remarkable Hatteria of "New Zealand, and have also some
resemblances to the Dinosaurians — reptiles which in some
respects approach birds. Considering how comparatively few
are the remains of this group of animals, the evidence which it
affords of progressive development is remarkably clear.1

Among the higher animals the rhinoceros, the horse, and
the deer afford good evidence of advance in organisation and
of the filling up of the gaps which separate the living forms
from their nearest allies. The earliest ancestral forms of the
rhinoceroses occur in the Middle Eocene of the United States,
and were to some extent intermediate between the rhinoceros
and tapir families, having like the latter four toes to the front
feet, and three to those behind. These are followed in the
Upper Eocene by the genus Amynodon, in which the skull
assumes more distinctly the rhinocerotic type. Following this
in the Lower Miocene we have the Aceratherium, like the last
in its feet, but still more decidedly a rhinoceros in its general
structure. From this there are two diverging lines — one in
the Old World, the other in the New. In the former, to which
the Aceratherium is supposed to have migrated in early
Miocene times, when a mild climate and luxuriant vegetation
prevailed far within the arctic circle, it gave rise to the
Ceratorhinus and the various horned rhinoceroses of late
Tertiary times and of those now living. In America a

1 On " Stagonolepis Robertsoni and on the Evolution of the Crocodilia, " in
Q. J. of Geological Society, 1875 ; and abstract in Nature, vol. xii. p. 38.

384 DARWINISM

number of large hornless rhinoceroses were developed —
they are found in the Upper Miocene, Pliocene, and Post-
Pliocene formations — and then became extinct. The true
rhinoceroses have three toes on all the feet.1

The Pedigree of the Horse Tribe.

Yet more remarkable is the evidence afforded by the
ancestral forms of the horse tribe which have been discovered
in the American tertiaries. The family Equidse, comprising
the living horse, asses, and zebras, differ widely from all other
mammals in the peculiar structure of the feet, all of which
terminate in a single large toe forming the hoof. They have
forty teeth, the molars being formed of hard and soft material
in crescentic folds, so as to be a powerful agent in grinding
up hard grasses and other vegetable food. The former peculi-
arities depend upon modifications of the skeleton, which have
been thus described by Professor Huxley : — ■

" Let us turn in the first place to the fore-limb. In most
quadrupeds, as in ourselves, the fore-arm contains distinct
bones, called the radius and the ulna. The corresponding
region in the horse seems at first to possess but one bone.
Careful observation, however, enables us to distinguish in this
bone a part which clearly answers • to the upper end of the
ulna. This is closely united with the chief mass of the bone
which represents the radius, and runs out into a slender shaft,
which may be traced for some distance downwards upon the
back of the radius, and then in most cases thins out and
vanishes. It takes still more trouble to make sure of what is
nevertheless the fact, that a small part of the lower end of the
bone of a horse's fore-arm, which is only distinct in a very
young foal, is really the lower extremity of the ulna.

"What is commonly called the knee of a horse is its wrist.
The ' cannon bone ' answers to the middle bone of the five
metacarpal bones Avhich support the palm of the hand in our-
selves. The pastern, coronary, and coffin bones of veterin-
arians answer to the joints of our middle fingers, while the
hoof is simply a greatly enlarged and thickened nail. But if

1 From a paper by Messrs. Scott and Osborne, "On the Origin and
Development of the Ehinoceros Group," read before the British Association
in 1883.

xiii THE GEOLOGICAL EVIDENCES OF EVOLUTION 385

what lies below the horse's ' knee ' thus corresponds to the
middle finger in ourselves, what has become of the four other
fingers or digits ? We find in the places of the second and
fourth digits only two slender splintlike bones, about two-
thirds as long as the cannon bone, which gradually taper to
their lower ends and bear no finger joints, or, as they are
termed, phalanges. Sometimes, small bony or gristly nodules
are to be found at the bases of these two metacarpal splints,
and it is probable that these represent rudiments of the first
and fifth toes. Thus, the part of the horse's skeleton which
corresponds with that of the human hand, contains one over-
grown middle digit, and at least two imperfect lateral digits ;
and these answer, respectively, to the third, the second, and
the fourth fingers in man.

" Corresponding modifications are found in the hind limb.
In ourselves, and in most quadrupeds, the leg contains two
distinct bones, a large bone, the tibia, and a smaller and more
slender bone, the fibula. But, in the horse, the fibula seems,
at first, to be reduced to its upper end ; a short slender bone
united with the tibia, and ending in a point below, occupying
its place. Examination of the lower end of a young foal's
shin-bone, however, shows a distinct portion of osseous matter
which is the lower end of the fibula ; so that the, apparently
single, lower end of the shin-bone is really made up of the
coalesced ends of the tibia and fibula, just as the, apparently
single, lower end of the forearm bone is composed of the coal-
esced radius and ulna.

" The heel of the horse is the part commonly known as
the hock. The hinder cannon bone answers to the middle
metatarsal bone of the human foot, the pastern, coronary,
and coffin bones, to the middle toe bones ; the hind hoof to
the nail ; as in the forefoot. And, as in the forefoot, there
are merely two splints to represent the second and the fourth
toes. Sometimes a rudiment of a fifth toe appears to be
traceable. .

" The teeth of a horse are not less peculiar than its limbs.
The living engine, like all others, must be well stoked if it is
to do its work ; and the horse, if it is to make good its wear
and tear, and to exert the enormous amount of force required
for its propulsion, must be well and rapidly fed. To this end,

2 c

386 DARWINISM

good cutting instruments and powerful and lasting crushers
are needful. Accordingly, the twelve cutting teeth of a horse
are close-set and concentrated in the forepart of its mouth,
like so many adzes or chisels. The grinders or molars are
large, and have an extremely complicated structure, being
composed of a number of different substances of unequal hard-
ness. The consequence of this is that they wear away at
different rates ; and, hence, the surface of each grinder is
always as uneven as that of a good millstone." -1

We thus see that the Equidse differ very widely in structure
from most other mammals. Assuming the truth of the theory
of evolution, we should expect to find traces among extinct
animals of the steps by which this great modification has
been effected ; and we do really find traces of these steps,
imperfectly among European fossils, but far more completely
among those of America.

It is a singular fact that, although no horse inhabited
America when discovered by Europeans, yet abundance of
remains of extinct horses have been found both in North and
South America in Post-Tertiary and Upper Pliocene deposits ;
and from these an almost continuous series of modified forms
can be traced in the Tertiary formation, till Ave reach, at
the very base of the series, a primitive form so unlike our
perfected animal, that, had we not the intermediate links, few
persons would believe that the one was the ancestor of the
other. The tracing out of this marvellous history we owe
chiefly to Professor Marsh of Yale College, who has himself
discovered no less than thirty species of fossil Equidse ; and
we will allow him to tell the story of the development of the
horse from a humble progenitor in his own words.

" The oldest representative of the horse at present known
is the diminutive Eohippus from the Lower Eocene. Several
species have been found, all about the size of a fox. Like
most of the early mammals, these ungulates had forty-four
teeth, the molars with short crowns and quite distinct in form
from the premolars. The ulna and fibula were entire and
distinct, and there were four well-developed toes and a rudi-
ment of another on the forefeet, and three toes behind. In
the structure of the feet and teeth, the Eohippus unmistak-
1 American Addresses, pp. 73-76.

xin THE GEOLOGICAL EVIDENCES OF EVOLUTION 387

ably indicates that the direct ancestral line to the modern
horse has already separated from the other perissodactyles, or
odd-toed ungulates.

" In the next higher division of the Eocene another genus,
Orohippus, makes its appearance, replacing Eohippus, and
showing a greater, though still distant, resemblance to the
equine type. The rudimentary first digit of the forefoot has
disappeared, and the last premolar has gone over to the molar
series, Orohippus was but little larger than Eohippus, and
in most other respects very similar. Several species have
been found, but none occur later than the Upper Eocene.

" Near the base of the Miocene, we find a third closely allied
genus, Mesohippus, which is about as large as a sheep, and one
stage nearer the horse. There are only three toes and a
rudimentary splint on the forefeet, and three toes behind.
Two of -the premolar teeth are quite like the molars. The
ulna is no longer distinct or the fibula entire, and other
characters show clearly that the transition is advancing.

" In the Upper Miocene Mesohippus is not found, but in its
place a fourth form, Miohippus, continues the line. This
genus is near the Anchitherium of Europe, but presents
several important differences. The three toes in each foot
are more nearly of a size, and a rudiment of the fifth meta-
carpal bone is retained. All the known species of this genus
are larger than those of Mesohippus, and none of them pass
above the Miocene formation.

" The genus Protohippus of the Lower Pliocene is yet more
equine, and some of its species equalled the ass in size. There
are still three toes on each foot, but only the middle one,
corresponding to the single toe of the horse, comes to the
ground. This genus resembles most nearly the Hipparion of
Europe.

" In the Pliocene we have the last stage of the series before
reaching the horse, in the genus Pliohippus, which has lost
the small hooflets, and in other respects is very equine.
Only in the Upper Pliocene does the true Equus appear and
complete the genealogy of the horse, which in the Post-Tertiary
roamed over the whole of North and South America, and soon
after became extinct. This occurred long before the dis-
covery of the continent by Europeans, and no satisfactory

388

DARWINISM

Hind- Fore

Leg. Upper molar. Lower molar.

BECENT.
Equus.

PLIOCENE.
Plioliippus.

Protohippus
(Hipparion).

MIOCENE.

Mioliippus
(Anchitherium).

Mesohippus.

EOCENE.
Orohippus.

f^mm

ec

Fig. 33.— Geological development of the horse tribe (Eohippus since discovered).

xiii THE GEOLOGICAL EVIDENCES OF EVOLUTION 389

reason for the extinction has yet been given. Besides the
characters I have mentioned, there are many others in the
skeleton, skull, teeth, and brain of the forty or more inter-
mediate species, which show that the transition from the
Eocene Eohippus to the modern Equus has taken place in the
order indicated " x (see Fig. 33).

Well may Professor Huxley say that this is demonstrative
evidence of evolution ; the doctrine resting upon exactly as
secure a foundation as did the Copernican theory of the
motions of the heavenly bodies at the time of its promulga-
tion. Both have the same basis — the coincidence of the
observed facts with the theoretical requirements.

Development of Deer's Horns.

Another clear and unmistakable proof of evolution is
afforded by one of the highest and latest developed tribes of
mammals — the true deer. These differ from all other ruminants
in possessing solid deciduous horns which are always more or
less branched. They first appear in the Middle Miocene
formation, and continue down to our time ; and their develop-
ment has been carefully traced by Professor Boyd Dawkins,
who thus summarises his results : — ■

"In the middle stage of the Miocene the cervine antler
consists merely of a simple forked crown (as in Cervus dicro-
ceros), which increases in size in the Upper Miocene, although
it still remains small and erect, like that of the roe. In Cervus
Matheroni it measures 11 '4 inches, and throws off not more
than four tines, all small. The deer living in Auvergne in
the succeeding or Pliocene age, present us with another stage
in the history of antler development. There, for the first
time, we see antlers of the Axis and Rusa type, larger and
longer, and more branching than any antlers were before, and
possessing three or more well-developed tines. Deer of this
type abounded in Pliocene Europe. They belong to the
Oriental division of the Cerviclse, and their presence in Europe
confirms the evidence of the flora, brought forward by the
Comte de Saporta, that the Pliocene climate was warm.
They have probably disappeared from Europe in consequence

1 Lecture on the Introduction and Succession of Vertebrate Life in America,
Nature, vol. xvi. p. 471.

390 DARWINISM

of the lowering of the temperature in the Pleistocene age,
while their descendants have found a congenial home in the
warmer regions of Eastern Asia.

" In the latest stage of the Pliocene — the Upper Pliocene of
the Val d'Arno — the Cervus dicranios of Nesti presents us with
antlers much smaller than those of the Irish elk, but very
complicated in their branching. This animal survived into
the succeeding age, and is found in the pre-glacial forest
bed of Norfolk, being described by Dr. Falconer under the
name of Sedgwick's deer. The Irish elk, moose, stag, reindeer,
and fallow deer appear in Europe in the Pleistocene age, all
with highly complicated antlers in the adult, and the first
possessing the largest antlers yet known. Of these the Irish
elk disappeared in the Prehistoric age, after having lived in
countless herds in Ireland, while the rest have lived on into
our own times in Euro-Asia, and, with the exception of the
last, also in North America.

"From this survey it is obvious that the cervine antlers
have increased in size and complexity from the Mid-Miocene
to the Pleistocene age, and that their successive changes are
analogous to those which are observed in the development of
antlers in the living deer, which begin with a simple point,
and increase in number of tines till their limit of growth be
reached. In other words, the development of antlers indicated
at successive and widely-separated pages of the geological
record is the same as that observed in the history of a single
living species. It is also obvious that the progressive
diminution of size and complexity in the antlers, from the
present time back into the early Tertiary age, sIioavs that we
are approaching the zero of antler development in the Mid-
Miocene. No trace of any antler-bearing ruminant has been
met with in the lower Miocenes, either of Europe or the
United States."1

Progressive Brain-Development.

The three illustrations now given sufficiently prove that,

whenever the geological record approaches to completeness,

we have evidence of the progressive change of species in

definite directions, and from less developed to more de-

1 Nature, vol. xxv. p. 84.

xni THE GEOLOGICAL EVIDENCES OF EVOLUTION 391

veloped types — exactly such a change as we may expect to
find if the evolution theory be the true one. Many other
illustrations of a similar change could be given, but the
animal groups in which they occur being less familiar, the
details would be less interesting, and perhaps hardly intel-
ligible. There is, however, one very remarkable proof of
development that must be briefly noticed — that afforded by
the steady increase in the size of the brain. This may be
best stated in the words of Professor Marsh : —

"The real progress of mammalian life in America, from
the beginning of the Tertiary to the present, is well illus-
trated by the brain-growth, in which Ave have the key to
many other changes. The earliest known Tertiary mammals
all had very small brains, and in some forms this organ was
proportionally less than in certain reptiles. There was a
graduaL increase in the size of the brain during this period,
and it is interesting to find that this growth was mainly
confined to the cerebral hemispheres, or higher portion of the
brain. In most groups of mammals the brain has gradually
become more convoluted, and thus increased in quality as
well as quantity. In some also the cerebellum and olfactory
lobes, the lower parts of the brain, have even diminished in
size. In the long struggle for existence during Tertiary time
the big brains won, then as now ; and the increasing power
thus gained rendered useless many structures inherited from
primitive ancestors, but no longer adapted to new conditions."

This remarkable proof of development in the organ of
the mental faculties, forms a fitting climax to the evidence
already adduced of the progressive evolution of the general
structure of the body, as illustrated by the bony skeleton.
We now pass on to another class of facts equally suggestive
of evolution.

The Local Relations of Fossil and Living Animals.

If all existing animals have been produced from ancestral
forms — mostly extinct — under the law of variation and natural
selection, we may expect to find in most cases a close rela-
tion between the living forms of each country and those which
inhabited it in the immediately preceding epoch. But if
species have originated in some quite different way, either by

392 DARWINISM

any kind of special creation, or by sudden advances of organisa-
tion in the offspring of preceding types, such close relationship
would not be found ; and facts of this kind become, therefore,
to some extent a test of evolution under natural selection or
some other law of gradual change. Of course the relationship
will not appear when extensive migration has occurred, by
which the inhabitants of one region have been able to take
possession of another region, and destroy or drive out its
original inhabitants, as has sometimes happened. But such
cases are comparatively rare, except where great changes of
climate are known to have occurred ; and we usually do find
a remarkable continuity between the existing fauna and flora
of a country and those of the immediately preceding age. A
few of the more remarkable of these cases will now be briefly
noticed.

The mammalian fauna of Australia consists, as is well
known, wholly of the lowest forms — the Marsupials and Mono-
tremata — except only a few species of mice. This is accounted
for by the complete isolation of the country from the Asiatic
continent during the whole period of the development of the
higher animals. At some earlier epoch the ancestral mar-
supials, which abounded both in Europe and North America
in the middle of the Secondary period, entered the country,
and have since remained there, free from the competition of
higher forms, and have undergone a special development in
accordance with the peculiar conditions of a limited area.
While in the large continents higher forms of mammalia have
been developed, which have almost or wholly exterminated the
less perfect marsupials, in Australia these latter have become
modified into such varied forms as the leaping kangaroos, the
burrowing wombats, the arboreal phalangers, the insectivorous
bandicoots, and the carnivorous Dasyuridse or native cats,
culminating in the Thylacinus or " tiger- wolf " of Tasmania —
animals as unlike each other as our sheep, rabbits, squirrels,
and dogs, but all retaining the characteristic features of the
marsupial type.

Now in the caves and late Tertiary or Post-Tertiary deposits
of Australia the remains of many extinct mammalia have been
found, but all are marsupials. There are many kangaroos,
some larger than any living species, and others more allied to

xiir THE GEOLOGICAL EVIDENCES OF EVOLUTION 393

the tree-kangaroos of New Guinea ; a large wombat as large
as a tapir ; the Diprotodon, a thick-limbed kangaroo the size of
a rhinoceros or small elephant ; and a quite different animal,
the Nototherium, nearly as large. The carnivorous Thyla-
cinus of Tasmania is also found fossil ; and a huge phalanger,
Thylacoleo, the size of a lion, believed by Professor Owen
and by Professor Oscar Schmidt to have been equally carni-
vorous and destructive.1 Besides these, there are many other
species more resembling the living forms both in size and
structure, of which they may be, in some cases, the direct
ancestors. Two species of extinct Echidna, belonging to the
very low Monotremata, have also been found in New South
Wales.

Next to Australia, South America possesses the most re-
markable assemblage of peculiar mammals, in its numerous
Edentata^ — the sloths, ant-eaters, and armadillos ; its rodents,
such as the cavies and chinchillas ; its marsupial opossums, and
its quadrumana of the family Cebidae. Remains of extinct
species of all these have been found in the caves of Brazil, of
Post-Pliocene age ; while in the earlier Pliocene deposits of the
pampas many distinct genera of these groups have been found,
some of gigantic size and extraordinary form. There are
armadillos of many types, some being as large as elephants ;
gigantic sloths of the generaMegatherium, Megalonyx, Mylodon,
Lestodon, and many others ; rodents belonging to the American
families Cavidse and Chinchillidse ; and ungulates allied to the
llama ; besides many other extinct forms of intermediate types
or of uncertain affinities.2 The extinct Moas of New Zealand
— huge wingless birds allied to the living Apteryx — illustrate
the same general law.

The examples now quoted, besides illustrating and enforcing
the general fact of evolution, throw some light on the usual
character of the modification and progression of animal forms.
In the cases where the geological record is tolerably complete,
we find a continuous development of some kind — either in
complexity of ornamentation, as in the fossil Paludinas of the
Hungarian lake-basins ; in size and in the specialisation of the

1 See The Mammalia in their Relation to Primeval Times, p. 102.

2 For a brief enumeration and description of these fossils, see the author's
Geographical Distribution of Animals, vol. i. p. 146.

394 DARWINISM

feet and teeth, as in the American fossil horses; or in the in-
creased development of the branching horns, as in the true
deer. In each of these cases specialisation and adaptation to
the conditions of the environment appear to have reached their
limits, and any change of these conditions, especially if it be
at all rapid or accompanied by the competition of less developed
but more adaptable forms, is liable to cause the extinction of
the most highly developed groups. Such we know was the
case with the horse tribe in America, which totally disappeared
in that continent at an epoch so recent that we cannot be
sure that the disappearance was not witnessed, perhaps caused,
by man; while even in the Eastern hemisphere it is the
smaller species — the asses and the zebras — that have persisted,
while the larger and more highly developed true horses have
almost, if not quite, disappeared in a state of nature. So we
find, both in Australia and South America, that in a quite
recent period many of the largest and most specialised forms
have become extinct, while only the smaller types have sur-
vived to our day ; and a similar fact is to be observed in many
of the earlier geological epochs, a group progressing and reach-
ing a maximum of size or complexity and then dying out,
or leaving at most but few and pigmy representatives.

Cause of Extinction of Large Animals.

Now there are several reasons for the repeated extinction
of large rather than of small animals. In the first place,
animals of great bulk require a proportionate supply of food,
and any adverse change of conditions would affect them more
seriously than it would smaller animals. In the next place,
the extreme specialisation of many of these large animals
would render it less easy for them to be modified in any new
direction suited to changed conditions. Still more important,
perhaps, is the fact that very large animals always increase
slowly as compared with small ones — the elephant producing
a single young one every three years, while a rabbit may have
a litter of seven or eight young two or three times a year.
Now the probability of favourable variations will be in direct
proportion to the population of the species, and as the smaller
animals are not only many hundred times more numerous than
the largest, but also increase perhaps a hundred times as

xin THE GEOLOGICAL EVIDENCES OF EVOLUTION 395

rapidly, they are able to become quickly modified by variation
and natural selection in harmony with changed conditions,
while the large and bulky species, being unable to vary quickly
enough, are obliged to succumb in the struggle for exist-
ence. As Professor Marsh well observes : "In every vigorous
primitive type which was destined to survive many geological
changes, there seems to have been a tendency to throw off
lateral branches, which became highly specialised and soon
died out, because they were unable to adapt themselves to new
conditions." And he goes on to show how the whole narrow
path of the persistent Suilline type, throughout the entire
series of the American tertiaries, is strewed with the remains of
such ambitious offshoots, many of them attaining the size of
a rhinoceros ; " while the typical pig, with an obstinacy never
lost, has held on in spite of catastrophes and evolution, and still
lives in America to-clay."

Indications of General Progression in Plants and Animals.

One of the most powerful arguments formerly adduced
against evolution was, that geology afforded no evidence of
the gradual development of organic forms, but that whole
tribes and classes appeared suddenly at definite epochs, and
often in great variety and exhibiting a very perfect organisa-
tion. The mammalia, for example, were long thought to have
first appeared in Tertiary times, where they are represented in
some of the earlier deposits by all the great divisions of the
class fully developed — carnivora, rodents, insectivora, mar-
supials, and even the perissodactyle and artiodactyle divisions of
the ungulata — as clearly defined as at the present day. The
discovery in 1818 of a single lower jaw in the Stonesfield
Slate of Oxfordshire hardly threw doubt on the generalisation,
since either its mammalian character was denied, or the
geological position of the strata, in which it was found, was
held to have been erroneously determined. But since then, at
intervals of many years, other remains of mammalia have been
discovered in the Secondary strata, ranging from the Upper
Oolite to the Upper Trias both in Europe and the United
States, and one even (Tritylodon) in the Trias of South Africa.
All these are either marsupials, or of some still lower type of
mammalia ; but they consist of many distinct forms classed in

396 DARWINISM

about twenty genera. Nevertheless, a great gap still exists
between these mammals and those of the Tertiary strata, since
no mammal of any kind has been found in any part of the
Cretaceous formation, although in several of its subdivisions
abundance of land plants, freshwater shells, and air-breathing
reptiles have been discovered. So with fishes. In the last
century none had been obtained lower than the Carboniferous
formation ; thirty years later they were found to be very
abundant in the Devonian rocks, and later still they were
discovered in the Upper Ludlow and Lower Ludlow beds of
the Silurian formation.

We thus see that such sudden appearances are deceptive,
and are, in fact, only what we ought to expect from the known
imperfection of the geological record. The conditions favour-
able to the fossilisation of any group of animals occur com-
paratively rarely, and only in very limited areas ; while the
conditions essential for their permanent preservation in the
rocks, amid all the destruction caused by denudation or meta-
morphism, are still more exceptional. And when they are
thus preserved to our day, the particular part of the rocks in
which they lie hidden may not be on the surface but buried
down deep under other strata, and may thus, except in the
case of mineral -bearing deposits, be altogether out of our
reach. Then, again, how large a proportion of the earth
consists of "wild and uncivilised regions in which no exploration
of the rocks has been yet made, so that whether we shall find
the fossilised remains of any particular group of animals
which lived during a limited period of the earth's history, and
in a limited area, depends upon at least a fivefold combination
of chances. Now, if we take each of these chances separately
as only ten to one against us (and some are certainly more
than this), then the actual chance against our finding the
fossil remains, say of any one order of mammalia, or of land
plants, at any particular geological horizon, will be about a
hundred thousand to one.

It may be said, if the chances are so great, how is it that
Ave find such immense numbers of fossil species exceeding in
number, in some groups, all those that are now living? But
this is exactly what we should expect, because the number of
species of organisms that have ever lived upon the earth, since

xni THE GEOLOGICAL EVIDENCES OF EVOLUTION 397

the earliest geological times, will probably be many hundred
times greater than those now existing of which we have any
knowledge ; and hence the enormous gaps and chasms in the
geological record of extinct forms is not to be wondered at.
Yet, notwithstanding these chasms in our knowledge, if
evolution is true, there ought to have been, on the whole,
progression in all the chief types of life. The higher and more
specialised forms should have come into existence later than the
lower and more generalised forms ; and however fragmentary
the portions we possess of the whole tree of life upon the
earth, they ought to show us broadly that such a progressive
evolution has taken place. We have seen that in some special
groups, already referred to, such a progression is clearly
visible, and we will now cast a hasty glance over the entire
series of fossil forms, in order to see if a similar progression is
manifested by them as a whole.

The Progressive Development of Plants.

Ever since fossil plants have been collected and studied, the
broad fact has been apparent that the early plants — those of
the Coal formation — were mainly cryptogamous, while in the
Tertiary deposits the higher flowering plants prevailed. In the
intermediate secondary epoch the gymnosperms — cycacls and
coniferse — formed a prominent part of the vegetation, and as
these have usually been held to be a kind of transition form
between the flowerless and flowering plants, the geological
succession has always, broadly speaking, been in accordance
with the theory of evolution. Beyond this, however, the facts
were very puzzling. The highest cryptogams — ferns, lycopods,
and equisetacese — appeared suddenly, and in immense profusion
in the Coal formation, at which period they attained a develop-
ment they have never since surpassed or even equalled ; while
the highest plants — the dicotyledonous and monocotyledonous
angiosperms — which now form the bulk of the vegetation of
the world,- and exhibit the most wonderful modifications of
form and structure, were almost unknown till the Tertiary
period, when they suddenly appeared in full development, and,
for the most part, under the same generic forms as now exist.

During the latter half of the present century, however,
great additions have been made to our knowledge of fossil

398 DARWINISM

plants ; and although there are still indications of vast gaps in
our knowledge, due, no doubt, to the very exceptional conditions
required for the preservation of plant remains, we now possess
evidence of a more continuous development of the various
types of vegetation. According to Mr. Lester F. Ward,
between 8000 and 9000 species of fossil plants have been
described or indicated; and, owing to the careful study
of the nervation of leaves, a large number of these are
referable to their proper orders or genera, and therefore give
us some notion — which, though very imperfect, is probably
accurate in its main outlines — of the progressive development
of vegetation on the earth.1 The following is a summary of
the facts as given by Mr. Ward : —

The lowest forms of vegetable life— the cellular plants —
have been found in Lower Silurian deposits in the form of three
species of marine algas ; and in the whole Silurian formation
fifty species have been recognised. We cannot for a moment
suppose, however, that this indicates the first appearance of
vegetable life upon the earth, for in these same Lower
Silurian beds the more highly organised vascular cryptogams
appear in the form of rhizocarps — plants allied to Marsilea
and Azolla, — and a very little higher, ferns, lycopods, and even
conifers appear. We have indications, however, of a still
more ancient vegetation, in the carbonaceous shales and thick
beds of graphite far down in the Middle Laurentian, since
there is no other known agency than the vegetable cell
by means of which carbon can be extracted from the atmo-

1 Sketch of Paleobotany in Fifth Annual Report of U. S. Geological Survey,
1883-84, pi?. 363-452, with diagrams. Sir J. William Dawson, speaking of
the value of leaves for the determination of fossil plants, says : "In my own
experience I have often found determinations of the leaves of trees confirmed
by the discovery of their fruits or of the structure of their stems. Thus, in
the rich cretaceous plant-beds of the Dunvegan series, we have beech-nuts
associated in the same bed with leaves referred to Fagus. In the Laramie
beds I determined many years ago nuts of the Trapa or water- chestnut, and
subsequently Lesquereux found in beds in the United States leaves which he
referred to the same genus. Later, I found in collections made on the Red Deer
River of Canada my fruits and Lesquereux's leaves on the same slab. The
presence of trees of the genera Carya and Juglans in the same formation was
inferred from their leaves, and specimens have since been obtained of silicified
wood with the microscopic structure of the modern butternut. Still we are
willing to admit that determinations from leaves alone are liable to doubt." —
The Geological History of Plants, p. 196,

xin THE GEOLOGICAL EVIDENCES OF EVOLUTION 399

sphere and fixed in the solid state. These great beds of
graphite, therefore, imply the existence of abundance of
vegetable life at the very commencement of the era of which
we have any geological record.1

Ferns, as already stated, begin in the Middle Silurian forma-
tion with the Eopteris Morrieri. In the Devonian, we have 79
species, in the Carboniferous 6 2 7, and in the Permian 186 species;
after which fossil ferns diminish greatly, though they are
found in every formation ; and the fact that fully 3000 living
species are known, while the richest portion of the Tertiary in
fossil plants — the Miocene — has only produced 87 species, will
serve to indicate the extreme imperfection of the geological
record.

The Equisetaceee (horsetails) which also first appear in
the Silurian and reach their maximum development in the
Coal formation, are, in all succeeding formations, far less
numerous than ferns, and only thirty living species are known.
Lycopodiacere, though still more abundant in the Coal form-
ation, are very rarely found in any succeeding deposit, though
the living species are tolerably numerous, about 500 having
been described. As we cannot suppose them to have really
diminished and then increased again in this extraordinary
manner, we have another indication of the exceptional nature
of plant preservation and the extreme and erratic character of
the imperfection of the record.

Passing now to the next higher division of plants — the
gymnosperms — we find Coniferse appearing in the Upper
Silurian, becoming tolerably abundant in the Devonian, and
reaching a maximum in the Carboniferous, from which form-
ation more than 300 species are known, equal to the
number recorded as now living. They occur in all succeeding
formations, being abundant in the Oolite, and excessively so
in the Miocene, from which 250 species have been described.
The allied family of gymnosperms, the Cycadaceae, first appear
in the Carboniferous era, but very scantily ; are most abundant
in the Oolite, from which formation 116 species are known,
and then steadily diminish to the Tertiary, although there are
seventy-five living species.

We now come to the true flowering plants, and we first
1 Sir J. William Dawson's Geological History of Plants, p. 18.

400 DARWINISM

meet with monocotyledons in the Carboniferous and Permian
formations. The character of these fossils was long disputed,
but is now believed to be well established ; and the sub-
class continues to be present in small numbers in all succeeding
deposits, becoming rather plentiful in the Upper Cretaceous,
and very abundant in the Eocene and Miocene. In the latter
formation 272 species have been discovered; but the 116
species in the Eocene form a larger proportion of the total
vegetation of the period.

True dicotyledons appear very much later, in the Cretaceous
period, and only in its upper division, if we except a single
species from the Urgonian beds of Greenland. The remark-
able thing is that we here find the sub-class fully developed
and in great luxuriance of types, all the three divisions —
Apetalse, Polypetalse, and Gamopetalse — being represented, with
a total of no less than 770 species. Among them are such
familiar forms as the poplar, the birch, the beech, the sycamore,
and the oak ; as well as the fig, the true laurel, the sassafras,
the j>ersimmon, the maple, the walnut, the magnolia, and even
the apple and the plum tribes. Passing on to the Tertiary
period the numbers increase, till they reach their maximum
in the Miocene, where more than 2000 species of dicoty-
ledons have been discovered. Among these the proportionate
number of the higher gamopetalse has slightly increased, but
is considerably less than at the present clay.

Possible Cause of sudden late Appearance of Exogens.

The sudden appearance of fully developed exogenous
flowering plants in the Cretaceous period is very analogous to
the equally sudden appearance of all the chief types of
placental mammalia in the Eocene ; and in both cases we
must feel sure that this suddenness is only apparent, due to
unknown conditions which have prevented their preservation
(or their discovery) in earlier formations. The case of the
dicotyledonous plants is in some respects the most extra-
ordinary, because in the earlier Mesozoic formations we appear
to have a fair representation of the flora of the period,
including such varied forms as ferns, equisetums, cycads,
conifers, and monocotyledons. The only hint at an explana-
tion of this anomaly has been given by Mr. Ball, who supposes

xiii THE GEOLOGICAL EVIDENCES OF EVOLUTION 401

that all these groups inhabited the lowlands, where there was
not only excessive heat and moisture, but also a super-
abundance of carbonic acid in the atmosphere — conditions
under which these groups had been developed, but which
were prejudicial to the dicotyledons. These latter are
supposed to have originated on the high table-lands and
mountain ranges, in a rarer and drier atmosphere in which
the quantity of carbonic acid gas was much less ; and any
deposits formed in lake beds at high altitudes and at such a
remote epoch have been destroyed by denudation, and hence
we have no record of their existence.1

During a few weeks spent recently in the Rocky Mountains,
I was struck by the great scarcity of monocotyledons and
ferns in comparison with dicotyledons — a scarcity due
apparently to the dryness and rarity of the atmosphere
favouring the higher groups. If we compare Coulter's Rocky
Mountain Botany with Gray's Botany of the Northern {East)
United States, we have two areas which differ chiefly in the
points of altitude and atmospheric moisture. Unfortunately,
in neither of these works are the species consecutively
numbered ; but by taking the pages occupied by the two
divisions of dicotyledons on the one hand, monocotyledons
and ferns on the other, we can obtain a good approximation.
In this way we find that in the flora of the North-Eastern
States the monocotyledons and ferns are to the dicotyledons in
the proportion of 45 to 100 ; in the Rocky Mountains they
are in the proportion of only 34 to 100 ; while if we take an
exclusively Alpine flora, as given by Mr. Ball, there are not
one-fifth as many monocotyledons as dicotyledons. These
facts show that even at the present day elevated plateaux
and mountains are more favourable to dicotyledons than to
monocotyledons, and we may, therefore, well suppose that the
former originated within such elevated areas, and were for
long ages confined to them. It is interesting to note that their
richest early remains have been found in the central regions
of the North American continent, where they now, proportion-
ally, most abound, and where the conditions of altitude and a
dry atmosphere were probably present at a very early period.

1 " On the Origin of the Flora of the European Alps," Proc, of Roy, Geog.
Society, vol. i. (1879), pp. 564-588.

2 D

402

DARWINISM

CHAP.

The diagram (Fig. 34), slightly modified from one given

by Mr. Ward, will illustrate our present knowledge of the
development of the vegetable kingdom in geological time.

xin THE GEOLOGICAL EVIDENCES OF EVOLUTION 403

The shaded vertical bands exhibit the proportions of the fossil
forms actually discovered, while the outline extensions are
intended to show what we may fairly presume to have been
the approximate periods of origin, and progressive increase of
the number of species, of the chief divisions of the vegetable
kingdom. These seem to accord fairly well with their respec-
tive grades of development, and thus offer no obstacle to the
acceptance of the belief in their progressive evolution.

Geological Distribution of Insects.

The marvellous development of insects into such an endless
variety of forms, their extreme specialisation, and their adapta-
tion to almost every possible condition of life, would almost
necessarily imply an extreme antiquity. Owing, however, to
their small size, their lightness, and their usually aerial habits,
no class' of animals has been so scantily preserved in the
rocks ; and it is only recently that the whole of the scattered
material relating to fossil insects and their allies have been
brought together by Mr. Samuel H. Scudder of Boston, and
we have thus learned their bearing on the theory of evolution.1

The most striking fact which presents itself on a glance at
the distribution of fossil insects, is the completeness of the
representation of all the chief types far back in the Secondary
period, at which time many of the existing families appear to
have been perfectly differentiated. Thus in the Lias we find
dragonflies "apparently as highly specialised as to-day, no
less than four tribes being present." Of beetles Ave have
undoubted Curculionidse from the Lias and Trias ; Chrysome-
lidee in the same deposits ; Cerambycidse in the Oolites ;
Scarabseidee in the Lias ; Buprestidse in the Trias ; Elateridse,
Trogositidse, and Nitidulidfe in the Lias ; Staphylinidae in the
English Purbecks; while Hydrophilidse, Gyrinidse, and Carabidae
occur in the Lias. All these forms are well represented, but
there are many other families doubtfully identified in equally
ancient rocks. Diptera of the families Empidse, Asilidse,
and Tipulidae have been found as far back as the Lias.
Of Lepidoptera, Sphingidse and Tineidas have been found

1 Systematic Review of our Present Knowledge of Fossil Insects, including
Myriapods and Arachnids {Bull, of U. S. Geol. Survey, No. 31, Washington,

404 DARWINISM

in the Oolite ; while ants, representing the highly specialised
Hymenoptera, have occurred in the Purbeck and Lias.

This remarkable identity of the families of very ancient
with those of existing insects is quite comparable with the
apparently sudden appearance of existing genera of trees in
the Cretaceous epoch. In both cases we feel certain that we
must go very much farther back in order to find the ancestral
forms from which they Avere developed, and that at any
moment some fresh discovery may revolutionise our ideas as
to the antiquity of certain groups. Such a discovery was
made while Mr. Scudder's work was passing through the press.
Up to that date all the existing orders of true insects appeared
to have originated in the Trias, the alleged moth and beetle of
the Coal formation having been incorrectly determined. But
now, undoubted remains of beetles have been found in the Coal
measures of Silesia, thus supporting the interpretation of the
borings in carboniferous trees as having been made by insects
of this order, and carrying back this highly specialised form of
insect life well into Palaeozoic times. Such a discovery renders
all speculation as to the origin of true insects premature,
because we may feel sure that all the other orders of insects,
except perhaps hymenoptera and lepidoptera, were contempo-
raneous with the highly specialised beetles.

The less highly organised terrestrial arthropoda — the
Arachnida and Myriapoda — are, as might be expected, much
more ancient. A fossil spider has been found in the Carboni-
ferous, and scorpions in the Upper Silurian rocks of Scotland,
Sweden, and the United States. Myriapoda have been found
abundantly in the Carboniferous and Devonian formations ;
but all are of extinct orders, exhibiting a more generalised
structure than living forms.

Much more extraordinary, however, is the presence in the
Palaeozoic formations of ancestral forms of true insects, termed
by Mr. Scudder Palseodictyoptera. They consist of general-
ised cockroaches and walking-stick insects (Orthopteroidea) ;
ancient mayflies and allied forms, of which there are six
families and more than thirty genera (Neuropteroidea) ; three
genera of Hemipteroidea resembling various Homoptera and
Hemiptera, mostly from the Carboniferous formation, a few
from the Devonian, and one ancestral cockroach (Paloeoblattina)

xin THE GEOLOGICAL EVIDENCES OF EVOLUTION" 405

from the Middle Silurian sandstone of France. If this
occurrence of a true hexapod insect from the Middle Silurian
be really established, taken in connection with the well-
defined Coleoptera from the Carboniferous, the origin of the
entire group of terrestrial arthropoda is necessarily thrown
back into the Cambrian epoch, if not earlier. And this cannot
be considered improbable in view of the highly differentiated
land plants — ferns, equisetums, and lycopods — in the Middle or
Lower Silurian, and even a conifer (Cordaites Robbii) in the
Upper Silurian ; while the beds of graphite in the Laurentian
were probably formed from terrestrial vegetation.

On the whole, then, we may affirm that, although the
geological record of the insect life of the earth is exceptionally
imperfect, it yet decidedly supports the evolution hypothesis.
The most specialised order, Lepidoptera, is the most recent,
only dating back to the Oolite ; the Hymenoptera, Diptera,
and Homoptera go as far as the Lias • while the Orthoptera
and Neuroptera extend to the Trias. The recent discovery of
Coleoptera in the Carboniferous shows, however, that the
preceding limits are not absolute, and will probably soon be
overpassed. Only the more generalised ancestral forms of
winged insects have been traced back to Silurian time, and
along with them the less highly organised scorpions ; facts
which serve to show us the extreme imperfection of our
knowledge, and indicate possibilities of a world of terrestrial
life in the remotest Palaeozoic times.

Geological Succession of Vertebrata.

The lowest forms of vertebrates are the fishes, and these appear
first in the geological record in the Upper Silurian formation.
The most ancient known fish is a Pteraspis, one of the buck-
lered ganoids or plated fishes — by no means a very low type
— allied to the sturgeon (Accipenser) and alligator - gar
(Lepiclosteus), but, as a group, now nearly extinct. Almost
equally ancient are the sharks, which under various forms
still abound in our seas. We cannot suppose these to be nearly
the earliest fishes, especially as the two lowest orders, now
represented by the Amphioxus or lancelet and the lampreys,
have not yet been found fossil. The ganoids were greatly
developed in the Devonian era, and continued till the

406 DARWINISM

Cretaceous, when they gave way to the true osseous fishes,
which had first appeared in the Jurassic period, and have con-
tinued to increase till the present day. This much later
appearance of the higher osseous fishes is quite in accordance
with evolution, although some of the very lowest forms, the
lancelet and the lampreys, together with the archaic ceratodus,
have survived to our time.

The Amphibia, represented by the extinct labyrinthodons,
appear first in the Carboniferous rocks, and these peculiar forms
became extinct early in the Secondary period. The labyrin-
thodons were, however, highly specialised, and do not at all
indicate the origin of the class, which may be as ancient as the
lower forms of fishes. Hardly any recognisable remains of our
existing groups — the frogs, toads, and salamanders — are found
before the Tertiary period, a fact which indicates the extreme
imperfection of the record as regards this class of animals.

True reptiles have not been found till we reach the Per-
mian where Prohatteria and Proterosaurus occur, the former
closely allied to the lizard-like Sphenodon of New Zealand,
the latter having its nearest allies in the same group of
reptiles — Rhyncocephala, other forms of which occur in the
Trias. In this last-named formation the earliest crocodiles —
Phytosaurus (Belodon) and Stagonolepis occur, as well as the
earliest tortoises — Chelytherium, Proganochelys, and Psepho-
derma.1 Fossil serpents have been first found in the Cre-
taceous formation, but the conditions for the preservation of
these forms have evidently been unfavourable, and the record
is correspondingly incomplete. The marine Plesiosauri and
Ichthyosauri, the flying Pterodactyles, the terrestrial Iguan-
odon of Europe, and the huge Atlantosaurus of Colorado —
the largest land animal that has ever lived upon the earth 2 —
all belong to special developments of the reptilian type which
flourished during the Secondary epoch, and then became
extinct.

1 For the facts as to the early appearance of the above named groups of
reptiles I am indebted to Mr. R. Lydekker of the Geological Department of
the Natural History Museum.

2 According to Professor Marsh this creature was 50 or 60 feet long, and
when erect, at least 30 feet in height. It fed upon the foliage of the
mountain forests of the Cretaceous epoch, the remains of which are preserved
with it.

xii r THE GEOLOGICAL EVIDENCES OF EVOLUTION 407

Birds are among the rarest of fossils, due, no doubt, to their
aerial habits removing them from the ordinary dangers of
flood, bog, or ice which overwhelm mammals and reptiles, and
also to their small specific gravity which keeps them floating
on the surface of water till devoured. Their remains were long
confined to Tertiary deposits, where many living genera and
a few extinct forms have been found. The only birds yet
known from the older rocks are the toothed birds (Odontor-
nithes) of the Cretaceous beds of the United States, belong-
ing to two distinct families and many genera ; a penguin-like
form (Enaliornis) from the Upper Greensand of Cambridge ;
and the well-known long-tailed Archseopteryx from the Upper
Oolite of Bavaria. The record is thus imperfect and fragment-
ary in the extreme ; but it yet shows us, in the few birds dis-
covered in the older rocks, more primitive and generalised
types, while the Tertiary birds had already become specialised
like those living, and had lost both the teeth and the long-
vertebral tail, which indicate reptilian affinities in the earlier
ages.

Mammalia have been found, as already stated, as far back
as the Trias formation, in Europe in. the United States and
in South Africa, all being very small, and belonging either
to the Marsupial order, or to some still lower and more
generalised type, out of which both Marsupials and Insectivora
were developed. Other allied forms have been found in the
Lower and Upper Oolite both of Europe and the United States.
But there is then a great gap in the whole Cretaceous
formation, from which no mammal has been obtained, although
both in the Wealden and the Upper Chalk in Europe, and in
the Upper Cretaceous deposits of the United States an
abundant and well-preserved terrestrial flora has been dis-
covered. Why no mammals have left their remains here it is
impossible to say. We can only suppose that the limited
areas in which land plants have been so abundantly preserved,
did not present the conditions which are needed for the fossil-
isation and preservation of mammalian remains.

When we come to the Tertiary formation, we find mammals
in abundance ; but a wonderful change has taken place. The
obscure early types have disappeared, and we discover in their
place a whole series of forms belonging to existing orders,

408

DARWINISM

and even sometimes to existing families. Thus, in the Eocene
we have remains of the opossum family ; bats apparently
belonging to living genera ; rodents allied to the South
American cavies and to dormice and squirrels ; hoofed animals
belonging to the odd-toed and even-toed groups ; and an-
cestral forms of cats, civets, dogs, with a number of more
generalised forms of carnivora. Besides these there are
whales, lemurs, and many strange ancestral forms of pro-
boscidea.1

The great diversity of forms and structures at so remote
an epoch would require for their development an amount of
time, which, judging by the changes that have occurred in
other groups, would carry us back far into the Mesozoic
period. In order to understand why we have no record of
these changes in any part of the woi'ld, we must fall back
upon some such supposition as we made in the case of the
dicotyledonous plants. Perhaps, indeed, the two cases are
really connected, and the upland regions of the primeval world,
which saw the development of our higher vegetation, may
have also afforded the theatre for the gradual development
of the varied mammalian types which surprise us by their
sudden appearance in Tertiary times.

Notwithstanding these irregularities and gaps in the record,
the accompanying table, summarising our actual knowledge of
the geological distribution of the five classes of vertebrata,

Geological Distribution of Mammalia.

i

o

c

&

a

3
o

_C8

'3

0
o
>

PI

o

0

rt

A

+j

a

.5

ca

R

O

P-t

H

i-5

O

O

H

"Fishes .
Amphibia

Reptiles
Birds .
Mammalia .

1 For fuller details, see the author's Geographical Distribution of Animals,
and Heilprin's Geographical and Geological Distribution of Animals.

xrn THE GEOLOGICAL EVIDENCES OF EVOLUTION 409

exhibits a steady progression from lower to higher types,
excepting only the deficiency in the bird record which is
easily explained. The comparative perfection of type in
which each of these classes first appears, renders it certain that
the origin of each and all of them must be sought much
farther back than any records which have yet been discovered.
The researches of palaeontologists and embryologists indicate
a reptilian origin for birds and mammals, while reptiles and
amphibia arose, perhaps independently, from fishes.

Concluding Remarks.

The brief review we have now taken of the more suggestive
facts presented by the geological succession of organic forms,
is sufficient to show that most, if not all, of the supposed
difficulties which it presents in the way of evolution, are due
either to imperfections in the geological record itself, or to our
still very incomplete knowledge of what is really recorded in
the earth's crust. We learn, however, that just as discovery
progresses, gaps are filled up and difficulties disappear ; while,
in the case of many individual groups, we have already
obtained all the evidence of progressive development that can
reasonably be expected. We conclude, therefore, that the
geological difficulty has now disappeared ; and that this noble
science, when properly understood, affords clear and weighty
evidence of evolution.

CHAPTER XIV

FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION
AND HEREDITY

Fundamental difficulties and objections— Mr. Herbert Spencer's factors
of organic evolution — Disuse and effects of Avithdrawal of natural
selection — Supposed effects of disuse among wild animals — Difficulty
as to co-adaptation of parts by variation and selection — Direct action
of the environment — The American school of evolutionists — Origin
of the feet of the ungulates— Supposed action of animal intelligence —
Semper on the direct influence of the environment — Professor Geddes's
theory of variation in plants — Objections to the theory — On the
origin of spines — Variation and selection overpower the effects of use
and disuse— Supposed action of the environment in imitating varia-
tions— Weismann's theory of heredity — The cause of variation — The
non-heredity of acquired characters — The theory of instinct — Con-
cluding remarks.

Having now set forth and illustrated at some length the
most important of the applications of the development
hypothesis in the explanation of the broader and more
generally interesting phenomena presented by the organic
world, we propose to discuss some of the more fundamental
problems and difficulties which have recently been adduced
by eminent naturalists. It is the more necessary to do this,
because there is now a tendency to minimise the action of
natural selection in the production of organic forms, and to
set up in its place certain fundamental principles of variation
or laws of growth, which it is urged are the real originators
of the several lines of development, and of most of the variety
of form and structure in the vegetable and animal kingdoms.
These views have, moreover, been seized upon by popular
writers to throw doubt and discredit on the whole theory of

chap, xiv FUNDAMENTAL PROBLEMS 411

evolution, and especially on Darwin's presentation of that
theory, to the bewilderment of the general public, who are
quite unable to decide how far the new views, even if well
established, tend to subvert the Darwinian theory, or whether
they are really more than subsidiary parts of it, and quite
powerless without it to produce any effect whatever.

The writers whose special views we now propose to
consider are : (1) Mr. Herbert Spencer, on modification of
structures arising from modification of functions, as set forth
in his Factors of Organic Evolution. (2) Dr. E. D. Cope, who
advocates similar views in detail, in his work entitled The
Origin of the Fittest, and may be considered the head of a
school of American naturalists who minimise the agency of
natural selection. (3) Dr. Karl Semper, who has especially
studied the direct influence of the environment in the whole
animal -kingdom, and has set forth his views in a volume on
The Natural Conditions of Existence as they Affect Animal Life.
(4) Mr. Patrick Geddes, who urges that fundamental laws of
growth, and the antagonism of vegetative and reproductive
forces, account for much that has been imputed to natural
selection.

We will now endeavour to ascertain what are the more
important facts and arguments adduced by each of the above
writers, and how far they offer a substitute for the action of
natural selection ; having done which, a brief account will be
given of the views of Dr. Aug. Weismann, whose theory of
heredity will, if established, strike at the very root of the
arguments of the first three of the writers above referred to.

Mr. Herbert Spencer's Factors of Organic Evolution.

Mr. Spencer, while fully recognising the importance and
wide range of the principle of natural selection, thinks that
sufficient weight has not been given to the effects of use
and disuse as a factor in evolution, or to the direct action
of the environment in determining or modifying organic
structures. As examples of the former class of actions, he
adduces the decreased size of the jaws in the civilised races
of mankind, the inheritance of nervous disease produced by
overwork, the great and inherited development of the udders
in cows and goats, and the shortened legs, jaws, and snout in

412 DARWINISM

improved races of pigs — the two latter examples being quoted
from Mr. Darwin, — and other cases of like nature. As
examples of the latter, Mr. Darwin is again quoted as
admitting that there are many cases in which the action of
similar conditions -appears to have produced corresponding
changes in different species ; and we have a very elaborate
discussion of the direct action of the medium in modifying the
protoplasm of simple organisms, so as to bring about the
difference between the outer surface and the inner part that
characterises the cells or other units of which they are formed.
Now, although this essay did little more than bring together
facts which had been already adduced by Mr. Darwin or by
Mr. Spencer himself, and lay stress upon their importance, its
publication in a popular review was immediately seized upon
as "an avoAved and definite declaration against some of the
leading ideas on which the Mechanical Philosophy depends,"
and as being "fatal to the adequacy of the Mechanical
Philosophy as any explanation of organic evolution,"1 — an
expression of opinion which would be repudiated by every
Darwinian. For, even admitting the interpretation which Mr.
Spencer puts on the facts he adduces, they are all included in
the causes which Darwin himself recognised as having acted
in bringing about the infinitude of forms in the organic world.
In the concluding chapter of the Origin of Species he says :
" I have now recapitulated the facts and considerations which
have thoroughly convinced me that species have been modified
during a long course of descent. This has been effected
chiefly through the natural selection of numerous successive,
slight, favourable variations ; aided in an important manner
by the inherited effects of the use and disuse of parts ; and
in an unimportant manner — that is, in relation to adaptive
structures whether past or present, by the direct action of
external conditions, and by variations which seem to us, in
our ignorance, to arise spontaneously." This passage, sum-
marising Darwin's whole inquiry, and explaining his final
point of view, shows how very inaccurate may be the popular
notion, as expressed by the Duke of Argyll, of any supposed
additions to the causes of change of species as recognised
by Darwin.

1 See the Duke of Argyll's letter in Nature, vol. xxxiv. p. 336.

xiv FUNDAMENTAL PROBLEMS 413

But, as we shall see presently, there is now much reason
to believe that the supposed inheritance of acquired modifica-
tions— that is, of the effects of use and disuse, or of the direct
influence of the environment — is not a fact ; and if so, the very
foundation is taken away from the whole class of objections
on which so much stress is now laid. It therefore becomes
important to inquire whether the facts adduced by Darwin,
Spencer, and others, do really necessitate such inheritance, or
whether any other interpretation of them is possible. I
believe there is such an interpretation ; and we will first
consider the cases of disuse on which Mr. Spencer lays most
stress.

The cases Mr. Spencer adduces as demonstrating the effects
of disuse in diminishing the size and strength of organs are,
the diminished size of the jaws in the races of civilised men,
and the diminution of the muscles used in closing the jaws in
the case of pet -dogs fed for generations on soft food. He
argues that the minute reduction in any one generation could
not possibly have been useful, and, therefore, not the subject
of natural selection ; and against the theory of correlation of
the diminished jaw Avith increased brain in man, he urges that
there are cases of large brain development, accompanied by
jaws above the average size. Against the theory of economy
of nutrition in the case of the pet-clogs, he places the abundant
food of these animals which would render such economy need-
less.

But neither he nor Mr. Darwin has considered the effects
of the withdrawal of the action of natural selection in keep-
ing up the parts in question to their full dimensions, which,
of itself, seems to me quite adequate to produce the results
observed. Recurring to the evidence, adduced in Chapter III,
of the constant variation occurring in all parts of the organism,
while selection is constantly acting on these variations in
eliminating all that fall below the best working standard, and
preserving only those that are fully up to it; and, remembering
further, that, of the whole number of the increase produced
annually, only a small percentage of the best adapted can be
preserved, we shall see that every useful organ will be kept
up nearly to its higher limit of size and efficiency. Now Mr.
Galton has proved experimentally that, when any part has

414 DARWINISM

thus been increased (or diminished) by selection, there is in
the offspring a strong tendency to revert to a mean or average
size, which tends to check further increase. And this mean
appears to be, not the mean of the actual existing individuals
but a lower mean,' or that from which they had been recently
raised by selection.1 He calls this the law of "Regression
towards Mediocrity," and it has been proved by experiments
with vegetables and by observations on mankind. This regres-
sion, in every generation, takes place even when both parents
have been selected for their high development of the organ in
question ; but when there is no such selection, and crosses are
allowed among individuals of every grade of development, the
deterioration will be very rapid ; and after a time not only
will the average size of the part be greatly reduced, but the
instances of full development will become very rare. Thus
what Weismann terms " panmixia," or free intercrossing, will
co-ojDerate with Galton's law of "regression towards mediocrity,"
and the result will be that, whenever selection ceases to act on
any part or organ which has heretofore been kept up to a
maximum of size and efficiency, the organ in question will
rapidly decrease till it reaches a mean value considerably
below the mean of the progeny that has usually been produced
each year, and very greatly below the mean of that portion
which has survived annually ; and this will take place by the
general law of heredity, and quite irrespective of any use or
disuse of the part in question. Now, no observations have been
adduced by Mr. Spencer or others, showing that the average
amount of change supposed to be due to disuse is greater than
that due to the law of regression towards mediocrity ; while
even if it were somewhat greater, we can see many possible
contributory causes to its production. In the case of civilised
man's diminished jaw, there may well be some correlation
between the jaw and the brain, seeing that increased mental
activity would lead to the withdrawal of blood and of nervous
energy from adjacent parts, and might thus lead to diminished
growth of those parts in the individual. And in the case of
pet-dogs, the selection of small or short-headed individuals
would imply the unconscious selection of those with less
massive temporal muscles, and thus lead to the concomitant
1 Journal of the Anthropological Institute, vol. xv. pp. 246-260.

xiv FUNDAMENTAL PROBLEMS 415

reduction of those muscles. The amount of reduction observed
by Darwin in the wing-bones of domestic ducks and poultry,
and in the hind legs of tame rabbits, is very small, and is
certainly no greater than the above causes will well account
for; Avhile so many of the external characters of all our
domestic animals have been subject to long-continued artificial
selection, and we are so ignorant of the possible correlations
of different parts, that the phenomena presented by them
seem sufficiently explained without recurrence to the assump-
tion that any changes in the individual, clue to disuse, are
inherited by the offspring.

Supposed Effects of Disuse among Wild Animals.

It may be urged, however, that among wild animals we have
many undoubted results of disuse much more pronounced than
those among domestic kinds, results which cannot be explained
by the causes already adduced. Such are the reduced size of
the wings of many birds on oceanic islands ; the abortion of
the eyes in many cave animals, and in some which live under-
ground ; and the loss of the hind limbs in whales and in some
lizards. These cases differ greatly in- the amount of the re-
duction of parts which has taken place, and may be due to
different causes. It is remarkable that in some of the birds
of oceanic islands the reduction is little if any greater than in
domestic birds, as in the water-hen of Tristan d'Acunha. Now
if the reduction of wing were due to the hereditary effects of
disuse, we should expect a very much greater effect in a bird
inhabiting an oceanic island than in a domestic bird, where the
disuse has been in action for an indefinitely shorter period.
In the case of many other birds, however — as some of the New
Zealand rails and the extinct dodo of Mauritius — the wings
have been reduced to a much more rudimentary condition,
though it is still obvious that they were once organs of flight ;
and in these cases we certainly require some other causes than
those which have reduced the wings of our domestic fowls.
One such cause may have been of the same nature as that
Avhich has been so efficient in reducing the wings of the insects
of oceanic islands — the destruction of those which, during the
occasional use of their wings, were carried out to sea. This
form of natural selection may well have acted in the case of

416 DARWINISM chap.

birds whose powers of flight were already somewhat reduced,
and to whom, there being no enemies to escape from, their use
was only a source of danger. We may thus, perhaps, account
for the fact that many of these birds retain small but useless
wings with which' they never fly ; for, the wings having been
reduced to this functionless condition, no power could reduce
them further except correlation of growth or economy of
nutrition, causes which only rarely come into play.

The complete loss of eyes in some cave animals may,
perhaps, be explained in a somewhat similar way. When-
ever, owing to the total darkness, they became useless, they
might also become injurious, on account of their delicacy of
organisation and liability to accidents and disease ; in which
case natural selection would begin to act to reduce, and finally
abort them ; and this explains why, in some cases, the rudi-
mentary eye remains, although completely covered by a pro-
tective outer skin. Whales, like moas and cassowaries, carry
us back to a remote past, of whose conditions we know too
little for safe speculation. We are quite ignorant of the ances-
tral forms of either of these groups, and are therefore without
the materials needful for determining the steps by which the
change took place, or the causes which brought it about. 1

On a review of the various examples that have been given
by Mr. Darwin and others of organs that have been reduced
or aborted, there seems too much diversity in the results for
all to be due to so direct and uniform a cause as the individual
effects of disuse accumulated by heredity. For if that were
the only or chief efficient cause, and a cause capable of pro-
ducing a decided effect during the comparatively short period

1 The idea of the non-heredity of acquired variations was suggested by
the summary of Professor Weismann's views, in Nature, referred to later on.
But since this chapter was written I have, through the kindness of Mr. E. B.
Poulton, seen some of the proofs of the forthcoming translation of Weismann's
Essays on Heredity, in which he sets forth an explanation very similar
to that here given. On the difficult question of the almost entire disap-
pearance of organs, as in the limbs of snakes and of some lizards, he adduces
" a certain form of correlation, which Roux calls ' the struggle of the parts in
the organism,' " as playing an important part. Atrophy following disuse is
nearly always attended by the corresponding increase of other organs : blind
animals possess more developed organs of touch, hearing, and smell ; the loss
of power in the wings is accompanied by increased strength of the legs, etc.
Now as these latter characters, being useful, will be selected, it is easy to
understand that a congenital increase of these will be accompanied by a cor-

xiv FUNDAMENTAL PROBLEMS 417

of the existence of animals in a state of domestication, we
should expect to find that, in wild species, all nnused parts or
organs had been reduced to the smallest rudiments, or had
wholly disappeared. Instead of this we find various grades
of reduction, indicating the probable result of several distinct
causes, sometimes acting separately, sometimes in combination,
such as those we have already pointed out.

And if Ave find no positive evidence of disuse, acting by its
direct effect on the individual, being transmitted to the offspring,
still less can we find such evidence in the case of the use of
organs. For here the very fact of use, in a wild state, implies
utility, and utility is the constant subject for the action of
natural selection ; while among domestic animals those parts
which are exceptionally used are so used in the service of man,
and have, thus become the subjects of artificial selection.
Thus " the great and inherited development of the udders in
cows and goats," quoted by Spencer from Darwin, really affords
no proof of inheritance of the increase due to use, because,
from the earliest period of the domestication of these animals,
abundant milk-production has been highly esteemed, and has
thus been the subject of selection ; while there are no cases
among wild animals that may not be better explained by
variation and natural selection.

Difficulty as to Co-adaptation of Parts by Variation and Selection.

Mr. Spencer again brings forward this difficulty, as he
did in his Principles of Biology twenty -five years ago, and
urges that all the adjustments of bones, muscles, blood-vessels,
and nerves which would be required during, for example, the
development of the neck and fore-limbs of the giraffe, could

responding congenital diminution of the unused organ ; and in cases where
the means of nutrition are deficient, every diminution of these useless parts
will be a gain to the whole organism, and thus their complete disappearance
will, in some cases, be brought about directly by natural selection. This
corresponds with what we know of these rudimentary organs.

It must, however, be pointed out that the non-heredity of acquired char-
acters was maintained by Mr. Francis Galtou more than twelve years ago, on
theoretical considerations almost identical with those urged by Professor Weis-
mann ; while the insufficiency of the evidence for their hereditary trans-
mission was shown, by similar arguments to those used above and in the work
of Professor Weismann already referred to (see " A Theory of Heredity," in
Journ. Anthrop. Instit., vol. v. pp. 343-345).

2 E

418 DARWINISM

not have been effected by " simultaneous fortunate spontaneous
variations." But this difficulty is fully disposed of by the
facts of simultaneous variation adduced in our third chapter,
and has also been specially considered in Chapter VI, p. 127.
The best answer 'to this objection may, perhaps, be found in
the fact that the very thing said to be impossible by variation
and natural selection has been again and again effected by
variation and artificial selection. During the process of forma-
tion of such breeds as the greyhound or the bull-dog, of the
race-horse and cart-horse, of the fantail pigeon or the otter-
sheep, many co-ordinate adjustments have been produced ; and
no difficulty has occurred, whether the change has been effected
by a single variation — as in the last case named — or by slow
steps, as in all the others. It seems to be forgotten that most
animals have such a surplus of vitality and strength for all the
ordinary occasions of life that any slight superiority in one
part can be at once utilised ; while the moment any want of
balance occurs, variations in the insufficiently developed parts
will be selected to bring back the harmony of the whole
organisation. The fact that, in all domestic animals, variations
do occur, rendering them sAvifter or stronger, larger or smaller,
stouter or slenderer, and that such variations can be separately
selected and accumulated for man's purposes, is sufficient to
render it certain that similar or even greater changes may be
effected by natural selection, Avhich, as Darwin well remarks,
" acts on every internal organ, on every shade of constitu-
tutional difference, on the whole machinery of life." The
difficulty as to co-adaptation of parts by variation and natural
selection appears to me, therefore, to be a wholly imaginary
difficulty which has no place whatever in the operations of
nature.

Direct Action of the Environment.

Mr. Spencer's last objection to the wide scope given by
Darwinians to the agency of natural selection is, that organisms
are acted upon by the environment, which produces in them
definite changes, and that these changes in the individual are
transmitted by inheritance, and thus become increased in
successive generations. That such changes are produced in
the individual there is ample evidence, but that they are in-

xiv FUNDAMENTAL PROBLEMS 419

herited independently of any form of selection or of reversion
is exceedingly doubtful, and Darwin nowhere expresses him-
self as satisfied with the evidence. The two very strongest
cases he mentions are the twenty-nine species of American
trees which all differed in a corresponding way from their
nearest European allies ; and the American maize which
became changed after three generations in Europe. But in
the case of the trees the differences alleged may be partly due
to correlation with constitutional peculiarities dependent on
climate, especially as regards the deeper tint of the fading leaves
and the smaller size of the buds and seeds in America than in
Europe ; Avhile the less deeply toothed or serrated leaves in the
American species are, in our present complete ignorance of the
causes and uses of serration, quite as likely to be due to some
form of^adaptation as to any direct action of the climate.
Again, we are not told how many of the allied species do not
vary in this particular manner, and this is certainly an im-
portant factor in any conclusion we may form on the question.

In the case of the maize it appears that one of the more
remarkable and highly selected American varieties was culti-
vated in Germany, and in three years nearly all resem-
blance to the original parent was lost ; and in the sixth year
it closely resembled a common European variety, but was of
somewhat more vigorous growth. In this case no selection
appears to have been practised, and the effects may have been
due to that " reversion to mediocrity " which invariably occurs,
and is more especially marked in the case of varieties which
have been rapidly produced by artificial selection. It may be
considered as a partial reversion to the wild or unimproved
stock ; and the same thing would probably have occurred,
though perhaps less rapidly, in America itself. As this is
stated by Darwin to be the most remarkable case known to
him "of the direct and prompt action of climate on a plant,"
we must conclude that such direct effects have not been proved
to be accumulated by inheritance, independently of reversion
or selection.

The remaining part of Mr. Spencer's essay is devoted
to a consideration of the hypothetical action of the environ-
ment on the lower organisms which consist of simple cells or
formless masses of protoplasm ; and he shows with great

420 DARWINISM

elaboration that the outer and inner parts of these are
necessarily subject to different conditions ; and that the outer
actions of air or water lead to the formation of integuments,
and sometimes to other definite modifications of the surface,
whence arise permanent differences of structure. Although
in these cases also it is very difficult to determine how much
is due to direct modification by external agencies transmitted
and accumulated by inheritance, and how much to spontaneous
variations accumulated by natural selection, the probabilities
in favour of the former mode of action are here greater,
because there is no differentiation of nutritive and reproductive
cells in these simple organisms ; and it can be readily seen
that any change produced in the latter will almost certainly
affect the next generation.1 We are thus carried back almost
to the origin of life, and can only vaguely speculate on what
took place under conditions of which we know so little.

The American School of Evolutionists.

The tentative views of Mr. Spencer which we have just dis-
cussed, are carried much further, and attempts have been made
to work them out in great detail, by many American naturalists,
whose best representative is Dr. E. D. Cope of Philadelphia.2
This school endeavours to explain all the chief modifications
of form in the animal kingdom by fundamental laws of growth
and the inherited effects of use and effort, returning, in fact, to
the teachings of Lamarck as being at least equally important
Avith those of Darwin.

The following extract will serve to show the high position
claimed by this school as original discoverers, and as having
made important additions to the theory of evolution : —

" Wallace and Darwin have propounded as the cause of
modification in descent their law of natural selection. This
law has been epitomised by Spencer as the ' survival of the
fittest.' This neat expression no doubt covers the case, but it
leaves the origin of the fittest entirely untouched. Darwin
assumes a ' tendency to variation ' in nature, and it is plainly

1 This explanation is derived from Weismann's Theory of the Continuity
of the Germ-Plasm as summarised in Nature.

2 See a collection of his essays under the title, The Origin of the Fittest :
Essays on Evolution. D. Appleton and Co. New York. 1887.

xiv FUNDAMENTAL PROBLEMS 421

necessary to do this, in order that materials for the exercise
of a selection should exist. Darwin and Wallace's law is then
only restrictive, directive, conservative, or destructive of some-
thing already created. I propose, then, to seek for the origin-
ative laws by which these subjects are furnished ; in other
words, for the causes of the origin of the fittest." 1

Mr. Cope lays great stress on the existence of a special
developmental force termed "bathmism" or growth-force,
which acts by means of retardation and acceleration "without
any reference to fitness at all ;" that "instead of being controlled
by fitness it is the controller of fitness." He argues that " all
the characteristics of generalised groups from genera up (ex-
cepting, perhaps, families) have been evolved under the law of
acceleration and retardation," combined with some intervention
of natural selection ; and that specific characters, or species,
have been evolved by natural selection with some assistance
from the higher law. He, therefore, makes species and genera
two absolutely distinct things, the latter not developed out of
the former ; generic characters and specific characters are, in
his opinion, fundamentally different, and have had different
origins, and whole groups of species have been simultaneously
modified, so as to belong to another genus ; whence he thinks
it " highly probable that the same specific form has existed
through a succession of genera, and perhaps in different epochs
of geologic time."

Useful characters, he concludes, have been produced by the
special location of growth-force by use ; useless ones have been
produced by location of growth-force without the influence of
use. Another element which determines the direction of
growth-force, and which precedes use, is effort; and "it is
thought that effort becomes incorporated into the metaphysical
acquisitions of the parent, and is inherited with other meta-
physical qualities by the young, which, during the period of
growth, is much more susceptible to modifying influences, and
is likely to exhibit structural change in consequence." 2

From these few examples of their teachings, it is clear that

1 Origin of the Fittest, p. 174-

2 Ibid. p. 29. It may be here noted that Darwin found these theories
unintelligible. In a letter to Professor B. T. Morse in 1877, he writes :
"There is one point which I regret you did not make clear in your Ad-

422 DARWINISM

these American evolutionists have departed very widely from
the views of Mr. Darwin, and in place of the well-established
causes and admitted laws to which he appeals have introduced
theoretical conceptions which have not yet been tested by
experiments or facts, as well as metaphysical conceptions
which are incapable of proof. And when they come to
illustrate these views by an appeal to palaeontology or
morphology, we find that a far simpler and more complete
explanation of the facts is afforded by the established principles
of variation and natural selection. The confidence with which
these new ideas are enunciated, and the repeated assertion
that without them Darwinism is powerless to explain the
origin of organic forms, renders it necessary to bestow a little
more time on the explanations they give us of well-known
phenomena with Avhich, they assert, other theories are incom-
petent to grapple.

As examples of use producing structural change, Mr. Cope
adduces the hooked and toothed beaks of the falcons and the
butcher-birds, and he argues that the fact of these birds belong-
ing to widely different groups proves that similarity of use has
produced a similar structural result. But no attempt is made
to show any direct causal connection between the use of a bill
to cut or tear flesh and the development of a tooth on the
mandible. Such use might conceivably strengthen the bill
or increase its size, but not cause a special tooth-like outgrowth
which was not present in the ancestral thrush-like forms of
the butcher-bird. On the other hand, it is clear that any
variations of the bill tending towards a hook or tooth would give
the possessor some advantage in seizing and tearing its prey,
and would thus be preserved and increased by natural selection.
Again, Mr. Cope urges the effects of a supposed "law of polar
or centrifugal growth " to counteract a tendency to un-
symmetrical growth, where one side of the body is used more
than the other. But the undoubted hurtfulness of want of
symmetry in many important actions or functions would
rapidly eliminate any such tendency. When, however, it has

dress, namely, what is the meaning and importance of Professors Cope and
Hyatt's views on acceleration and retardation ? I have endeavoured, and
given np in despair, the attempt to grasp their meaning " {Life and Letters,
vol. iii. p. 233).

FUNDAMENTAL PROBLEMS 423

become useful, as in the case of the single enlarged claw of
many Crustacea, it has been preserved by natural selection.

Origin of the Feet of the Ungulates.

Perhaps the most original and suggestive of Mr. Cope's
applications of the theory of use and effort in modifying
structure are, his chapters "On the Origin of the Foot-Structure
of the Ungulates ; " and that " On the Effect of Impacts and
Strains on the Feet of Mammalia ; " and they will serve also
to show the comparative merits of this theory and that of
natural selection in explaining a difficult case of modification,
especially as it is an explanation claimed as new and
original when first enunciated in 1881. Let us, then, see
how he deals with the problem.

The remarkable progressive change of a four or five-toed
ancestor-into the one-toed horse, and the equally remarkable
division of the whole group of ungulate animals into the odd-
toed and even-toed divisions, Mr. Cope attempts to explain
by the effects of impact and use among animals which
frequented hard or swampy ground respectively. On hard
ground, it is urged, the long middle toe would be most
used and subjected to the greatest strains, and would
therefore acquire both strength and development. It would
then be still more exclusively used, and the extra nourish-
ment required by it would be drawn from the adjacent less-
used toes, which would accordingly diminish in size, till, after
a long series of changes, the records of which are so well
preserved in the American tertiary rocks, the true one-toed
horse was developed. In soft or swampy ground, on the other
hand, the tendency would be to spread out the foot so that
there were two toes on each side. The two middle toes
would thus be most used and most subject to strains, and
would, therefore, increase at the expense of the lateral toes.
There would be, no doubt, an advantage in these two func-
tional toes being of equal size, so as to prevent twisting of the
foot while walking ; and variations tending to bring this about
would be advantageous, and would therefore be preserved.
Thus, by a parallel series of changes in another direction,
adapted to a distinct set of conditions, we should arrive at the
symmetrical divided hoofs of our deer and cattle. The fact

424 DARWINISM

that sheep and goats are specially mountain and rock-loving
animals may be explained by their being a later modification,
since the divided hoof once formed is evidently well adapted
to secure a firm footing on rugged and precipitous ground,
although it could hardly have been first developed in such
localities. Mr. Cope thus concludes : " Certain it is that the
length of the bones in the feet of the ungulate orders has a
direct relation to the dryness of the ground they inhabit, and
the possibility of speed which their habit permits them or
necessarily imposes on them."1

If there is any truth in the explanation here briefly
summarised, it must entirely depend on the fact of individual
modifications thus produced being hereditary, and we yet
await the proof of this. In the meantime it is clear that the
very same results could have been brought about by variation
and natural selection. For the toes, like all other organs,
vary in size and proportions, and in their degree of union or
separation ; and if in one group of animals it was beneficial to
have the middle toe larger and longer, and in another set to
have the two middle toes of the same size, nothing can be
more certain than that these particular modifications would
be continuously preserved, and the very results we see ulti-
mately produced.

The oft-repeated objections that the cause of variations is
unknown, that there must be something to determine variations
in the right direction ; that " natural selection includes no
actively progressive principle, but must wait for the develop-
ment of variation, and then, after securing the survival of the
best, wait again for the best to project its own variations for
selection," we have already sufficiently answered by showing
that variation — in abundant or typical species — is always
present in ample amount; that it exists in all parts and
organs ; that these vary, for the most part, independently, so
that any required combination of variations can be secured ;
and finally, that all variation is necessarily either in excess or
defect of the mean condition, and that, consequently, the right
or favourable variations are so frequently present that the
unerring power of natural selection never wants materials to
work upon.

1 Origin of the Fittest, p. 374.

FUNDAMENTAL PROBLEMS 425

Supposed Action of Animal Intelligence.

The following passage briefly summarises Mr. Cope's
position : " Intelligence is a conservative principle, and
will ahvays direct effort and use into lines which will be
beneficial to its possessor. Here we have the source of the
fittest, i.e. addition of parts by increase and location of
growth-force, directed by the influence of various kinds of
compulsion in the lower, and intelligent option among higher
animals. Thus intelligent choice, taking advantage of the
successive evolution of physical conditions, may be regarded
as the originator of the fittest, while natural selection is the
tribunal to which all results of accelerated growth are sub-
mitted. This preserves or destroys them, and determines the
new points of departure on which accelerated growth shall
build." r'

This notion of " intelligence " — the intelligence of the
animal itself — determining its own variation, is so evidently a
very partial theory, inapplicable to the whole vegetable king-
dom, and almost so to all the lower forms of animals, amongst
which, nevertheless, there is the very same adaptation and
co-ordination of parts and functions as among the highest, that
it is strange to see it put forward with such confidence as
necessary for the completion of Darwin's theory. If "the
various kinds of compulsion " — by which are apparently meant
the laws of variation, growth, and reproduction, the struggle
for existence, and the actions necessary to preserve life under
the conditions of the animal's environment — are sufficient to
have developed the varied forms of the lower animals and of
plants, we can see no reason why the same " compulsion "
should not have carried on the development of the higher
animals also. The action of this " intelligent option " is alto-
gether unproved ; while the acknowledgment that natural
selection is the tribunal which either preserves or destroys the
variations submitted to it, seems quite inconsistent with the
statement that intelligent choice is the "orginator of the
fittest," since whatever is really "the fittest " can never be
destroyed by natural selection, which is but another name for
the survival of the fittest. If " the fittest " is always definitely

1 Origin of the Fittest, p. 40.

426

DARWINISM

produced by some other power, then natural selection is not
wanted. If, on the other hand, both fit and unfit are produced,
and natural selection decides between them, that is pure Dar-
winism, and Mr. Cope's theories have added nothing to it.

Fig. 35.— Transformation of Artemia salina to A. Milhausenii ; 1, tail-lobe of A. salina,
and its transition through 2, 3, 4, 5, to 6, into that of A. Milhausenii ; 7,
post-abdomen of A. salina ; 8, post-abdomen of a form bred in brackish
water ; 9, gill of A. Milhausenii ; 10, gill of A. salina. (From Schmanke-
witsch.)

Semper on the Direct Influence of the Environment.

Another eminent naturalist, Professor Karl Semper of
Wiirzburg, also adopts the view of the direct transforming
power of the environment, and has brought together an

FUNDAMENTAL PKOBLEMS

427

immense body of interesting facts showing the influence of
food, of light, of temperature, of still water and moving water,
of the atmosphere and its currents, of gravitation, and of other
organisms, in modifying the forms and other characteristics of
animals.1 He believes that these various influences produce
a direct and important effect, and that this effect is accumu-
lated by inheritance ; yet he acknowledges that we have no
direct evidence of this, and there is hardly a single case
adduced in the book which is not equally well explained by
adaptation, brought about by the survival of beneficial varia-
tions. Perhaps the most remarkable case he has brought
forward is that of the transformation of species of crustaceans
by a change in the saltness of the water (see Fig. 35). Artemia
salina lives in brackish water, while A. Milhausenii inhabits
Avater which is much Salter. They differ greatly in the form of
the tail-lobes, and in the presence or absence of spines upon the
tail, and had always been considered perfectly distinct species.
Yet either was transformed into the other in a few generations,
during which the saltness of the water was gradually altered.
Yet more, A. salina was gradually accustomed to fresher
water, and in the course of a few generations, when the water
had become perfectly fresh, the species
was changed into Branchipus stag-
nalis, Avhich had always been con-
sidered to belong to a different genus
on account of differences in the form
of the antennse and of the posterior
segments of the body (see Fig. 36).
This certainly appears to be a proof
of change of conditions producing
a change of form independently of
selection, and of that change of form,
while remaining under the same con-
ditions, being inherited. Yet there
is this peculiarity in the case, that
there is a chemical change in the water, and that this water
permeates the whole body, and must be absorbed by the
tissues, and thus affect the ova and even the reproductive

1 The Natural Conditions of Existence as they Affect Animal Life.
London, 1883.

Fig. 36.

a. Branchipus stagnalis.

b. Artemia salina.

428 DARWINISM

elements, and in this way may profoundly modify the whole
organisation. Why and how the external effects are limited
to special details of the structure we do not know ; but it does
not seem as if any far-reaching conclusions as to the cumula-
tive effect of external conditions on the higher terrestrial
animals and plants, can be drawn from such an exceptional
phenomenon. It seems rather analogous to those effects of
external influences on the very lowest organisms in which the
vegetative and reproductive organs are hardly differentiated,
in which case such effects are doubtless inherited.1

Professor Geddes's Theory of Variation in Plants.

In a paper read before the Edinburgh Botanical Society in
1886 Mr. Patrick Geddes laid down the outlines of a funda-
mental theory of plant variation, which he has further ex-
tended in the article "Variation and Selection" in the
Encyclopaedia Britannica, and in a paper read before the Linnsean
Society but not yet published.

A theory of variation should deal alike with the origin of
specific distinctions and with those vaster differences which
characterise the larger groups, and he thinks it should answer
such questions as — How an axis comes to be arrested to form
a flower 1 how the various forms of inflorescence were evolved 1
how did perigynous or epigynous flowers arise from hypogynous
flowers ? and many others equally fundamental. Natural selec-
tion acting upon numerous accidental variations will not, he
urges, account for such general facts as these, which must
dejjend on some constant law of variation. This law he
believes to be the well-known antagonism of vegetative and
reproductive growth acting throughout the whole course of
plant development; and he uses it to explain many of the
most characteristic features of the structure of flowers and
fruits.

1 In Dr. Weismarm's essay on " Heredity," already referred to,. he considers
it not improbable that changes in organisms produced by climatic influences
may be inherited, because, as these changes do not affect the external parts
of an organism only, but often, as in the case of warmth or moisture per-
meate the whole structure, they may possibly modify the germ -plasm
itself, and thus induce variations in the next generation. In this way, he
thinks, may possibly be explained the climatic varieties of certain butterflies,
aud some other changes which seem to be effected by change of climate in a
few generations.

xiv FUNDAMENTAL PROBLEMS 429

Commencing with the origin of the floAver, which all botanists
agree in regarding as a shortened branch, he explains this
shortening as an inevitable physiological fact, since the cost of
the development of the reproductive elements is so great as
necessarily to check vegetative growth. In the same manner
the shortening of the inflorescence from raceme to spike or
umbel, and thence to the capitulum or dense flower-head
of the composite plants is brought about. This shortening,
carried still further, produces the flattened leaf-like receptacle
of Dorstenia, and further still the deeply hollowed fruity
receptacle of the fig.

The flower itself undergoes a parallel modification due to a
similar cause. It is formed by a series of modified leaves
arranged round a shortened axis. In its, earlier stages the
number of these modified leaves is indefinite, as in many
Ranunculacese ; and the axis itself is not greatly shortened, as
in Myosurus. The first advance is to a definite number of
parts and a permanently shortened axis, in the arrangement
termed hypogynous, in which all the whorls are quite distinct
from each other. In the next stage there is a further shorten-
ing of the central axis, leaving the outer portion as a ring on
which the petals are inserted, producing the arrangement
termed perigynous. A still further advance is made by the
contraction of the axis, so as to leave the central part form-
ing the ovary quite below the flower, which is then termed
epigynous.

These several modifications are said to be parallel and
definite, and to be determined by the continuous checking of
vegetation by reproduction along what is an absolute groove
of progressive change. This being the case, the importance of
natural selection is greatly diminished. Instead of selecting
and accumulating spontaneous indefinite variations, its function
is to retard them after the stage of maximum utility has been
independently reached. The same simple conception is said
to unlock innumerable problems of vegetable morphology, large
and small alike. It explains the inevitable development of
gymnosperm into angiosperm by the checked vegetative growth
of the ovule-bearing leaf or carpel ; while such minor adapta-
tions as the splitting fruit of the geranium or the cupped stigma
of the pansy, can be no longer looked upon as achievements

430 DARWINISM

of natural selection, but must be regarded as naturally trace-
able to the vegetative checking of their respective types of
leaf organ. Again, a detailed examination of spiny plants
practically excludes the hypothesis of mammalian selection
altogether, and shows spines to arise as an expression of the
diminishing vegetativeness — in fact, the ebbing vitality of a
species.1

Objections to the Theory.

The theory here sketched out is enticing, and at first sight
seems calculated to throw much light on the history of plant
development ; but on further consideration, it seems wanting
in definiteness, while it is beset with difficulties at every step.
Take first the shortening of the raceme into the umbel and the
capitulum, said to be caused by arrest of vegetative growth,
due to the antagonism of reproduction. If this were the
Avhole explanation of the phenomenon, we should expect the
quantity of seed to increase as this vegetative growth dimin-
ished, since the seed is the product of the reproductive energy
of the plant, and its quantity the best measure of that energy.
But is this the case % The ranunculus has comparatively few
seeds, and the flowers are not numerous ; while in the same
order the larkspur and the columbine have far more seeds as
well as more flowers, but there is no shortening of the raceme
or diminution of the foliage, although the flowers are large and
complex. So, the extremely shortened and compressed flower-
heads of the compositse produce comparatively few seeds
— one only to each flower ; while the foxglove, Avith its long
spike of showy flowers, produces an enormous number.

Again, if the shortening of the central axis in the successive
stages of hypogynous, perigynous, and epigynous flowers were an
indication of preponderant reproduction and diminished vegeta-
tion, Ave should find everyAvhere some clear indications of this
fact. The plants with hypogynous floAvers should, as a rule,
have less seed and more vigorous and abundant foliage than
those at the other extreme Avith epigynous floAArers. But the

1 This brief indication of Professor Geddes's views is taken from the
article " Variation and Selection " in the Hncydojxedia Britannica, and a paper
" On the Nature and Causes of Variation in Plants " in Trans, and Proc. of the
Edinburgh Botanical Society, 1886 ; and is, for the most part, expressed in
his own words.

xiv FUNDAMENTAL PROBLEMS 431

hypogynous poppies, pinks, and St. John's worts have abund-
ance of seed and rather scanty foliage ; while the epigynous
dogwoods and honeysuckles have few seeds and abundant
foliage. If, instead of the number of the seeds, we take the
size of the fruit as an indication of reproductive energy, we find
this at a maximum in the gourd family, yet their rapid and
luxuriant growth shows no diminution of vegetative power.
So that the statement that plant modifications proceed " along
an absolute groove of progressive change " is contradicted by
innumerable facts indicating advance and regression, improve-
ment or degradation, according as the ever-changing environ-
ment renders one form more advantageous than the other.
As one instance I may mention the Anonacese or custard-apple
tribe, which are certainly an advance from the Ranunculacese ;
yet in the genus Polyalthea the fruit consists of a number of
separate carpels, each borne on a long stalk, as if reverting to
the primitive stalked carpellary leaves.

On the Origin of Spines.

But perhaps the most extraordinary application of the
theory is that which considers spines to be an indication of the
" ebbing vitality of a species," and which excludes " mammalian
selection altogether." If this were true, spines should occur
mainly in feeble, rare, and dying-out species, instead of which
we have the hawthorn, one of our most vigorous shrubs or trees,
with abundant vitality and an extensive range over the whole
Palaearctic region, showing that it is really a dominant species.
In North America the numerous thorny species of Crataegus
are equally vigorous, as are the false acacia (Robinia) and
the honey-locust (Gleditschia). Neither have the numerous
species of very spiny Acacias been noticed to be rarer or less
vigorous than the unarmed kinds.

On the other point — that spines are not due to mammalian
selection — we are able to adduce what must be considered direct
and conclusive evidence. For if spines, admittedly produced by
aborted branches, petioles, or peduncles, are due solely or mainly
to diminished vegetativeness or ebbing vitality, they ought to
occur in all countries alike, or at all events in all whose similar
conditions tend to check vegetation ; whereas, if they are,
solely or mainly, developed as a protection against the attacks

432 DARWINISM

of herbivorous mammals, they ought to be most abundant
where these are plentiful, and rare or absent where indigenous
mammalia are wanting. Oceanic islands, as compared with
continents, would thus furnish a crucial test of the two theories ;
and Mr. Hemsley of Kew, who has specially studied insular
floras, has given me some valuable information on this point.
He says : " There are no spiny or prickly plants in the in-
digenous element of the St. Helena flora. The relatively rich
flora of the Sandwich Isles is not absolutely without a prickly
plant, but almost so. All the endemic genera are unarmed,
and the endemic species of almost every other genus. Even such
genera as Zanthoxylon, Acacia, Xylosoma, Lycium, and Solanum,
of which there are many armed species in other countries,
are only represented by unarmed species. The two endemic
Kubi have the prickles reduced to the setaceous condition, and
the two palms are unarmed.

"The flora of the Galapagos includes a number of prickly
plants, among them several cacti (these have not been inves-
tigated and may be American species), but I do not think one
of the known endemic species of any family is prickly or
spiny.

" Spiny and prickly plants are also rare in New Zealand,
but there are the formidably armed species of wild Spaniard
(Aciphylla), one species of Rubus, the pungent-leaved Epacridese
and a few others."

Mr. J. G. Baker of Kew, who has specially studied the
flora of Mauritius and the adjacent islands, also Avrites me on
this point. He says : " Taking Mauritius alone, I do not
call to mind a single species that is a spinose endemic tree or
shrub. If you take the whole group of islands (Mauritius,
Bourbon, Seychelles, and Eoclriguez), there will be about a
dozen species, but then nine of these are palms. Leaving
out palms, the trees and shrubs of that part of the world are
exceptionally non-spinose."

These are certainly remarkable facts, and quite inexplicable
on the theory of spines being caused solely by checked vege-
tative growth, due to weakness of constitution or to an arid soil
and climate. For the Galapagos and many parts of the Sand-
wich Islands are very arid, as is a considerable part of the
North Island of New Zealand. Yet in our own moist climate

xiv FUNDAMENTAL PEOBLEMS 433

and with our very limited number of trees and shrubs we
have about eighteen spiny or prickly species, more, apparently,
than in the whole endemic floras of the Mauritius, Sandwich
Islands, and Galapagos, though these are all especially rich
in shrubby and arboreal species. In New Zealand the prickly
Rubus is a leafless trailing plant, and its prickles are probably
a protection against the large snails of the country, several of
which have shells from two to three and a half inches long.1
The " wild Spaniards " are very spiny herbaceous Umbelliferse,
and may have gained their spines to preserve them from being
trodden down or eaten by the Moas, which, for countless ages,
took the place of mammals in New Zealand. The exact use
or meaning of the spines in palms is more doubtful, though
they are, no doubt, protective against some animals ; but it is
certainly an extraordinary fact that in the entire flora of the
Mauritius, so largely consisting of trees and shrubs, not a
single endemic species should be thorny or spiny.

If now we consider that every continental flora produces
a considerable proportion of spiny and thorny species, and that
these rise to a maximum in South Africa, where herbivorous
mammalia were (before the settlement of the country), perhaps,
more abundant and varied than in any other part of the
world ; while another district, remarkable for well-armed
vegetation, is Chile, where the camel-like vicugnas, llamas, and
alpacas, and an abundance of large rodents wage perpetual
war against shrubby vegetation, we shall see the full signify
cance of the almost total absence of thorny and spiny plants in
the chief oceanic islands ; and so far from " excluding the
hypothesis of mammalian selection altogether," we shall find
in this hypothesis the only satisfactory explanation of the
facts.

From the brief consideration of Professor Gecldes's theory
now given, we conclude that, although the antagonism between
vegetative and reproductive growth is a real agency, and must
be taken account of in our endeavour to explain many of the
fundamental facts in the structure and form of plants, yet it
is so overpowered and directed at every step by the natural
selection of favourable variations, that the results of its

1 Plaeostylis bovinus, 3£ inches long ; Paryphanta Busbyi, 3 in. diam. ;
P. Hochstetteri, 2| in. diam.

2 F

434 DARWINISM

exclusive and unmodified action are nowhere to be found in
nature. It may be allowed to rank as one of those "laws of
growth," of which so many have now been indicated, and
which were always recognised by Darwin as underlying all
variation ; but unless we bear in mind that its action must
always be subordinated to natural selection, and that it is
continually checked, or diverted, or even reversed by the
necessity of adaptation to the environment, we shall be liable
to fall into such glaring errors as the imputing to "ebbing
vitality " alone such a widespread phenomenon as the occur-
rence of spines and thorns, while ignoring altogether the
influence of the organic environment in their production.1

The sketch now given of the chief attempts that have been
made to prove that either the direct action of the environment
or certain fundamental laws of variation are independent causes
of modification of species, shows us that their authors have,
in every case, failed to establish their contention. Any direct
action of the environment, or any characters acquired by use
or disuse, can have no effect whatever upon the race unless
they are inherited ; and that they are inherited in any case,

1 The general arguments and objections here set forth will apply with equal
force to Professor G. Henslow's theory of the origin of the various forms
and structures of flowers as due to ' ' the responsive actions of the protoplasm
in consequence of the irritations set up by the weights, pressures, thrusts,
tensions, etc., of the insect visitors" {The Origin of Floral Structures through
Insect and other Agencies, p. 340). On the assumption that acquired char-
acters are inherited, such irritations may have had something to do with
the initiation of variations and with the production of certain details of
structure, but they are clearly incompetent to have brought about the
more important structural and functional modifications of flowers. Such
are, the various adjustments of length and position of the stamens to bring
the pollen to the insect and from the insect to the stigma ; the various
motions of stamens and styles at the right time and the right direction ;
the physiological adjustments bringing about fertility or sterility in hetero-
styled plants ; the traps, springs, and complex movements of various parts
of orchids ; and innumerable other remarkable phenomena.

For the explanation of these we have no resource but variation and selec-
tion, to the effects of which, acting alternately with regression or degradation
as above explained (p. 328) must be imputed the development of the count-
less floral structures we now behold. Even the primitive flowers, whose
initiation may, perhaps, have been caused, or rendered possible, by the
irritation set up by insects' visits, must, from their very origin, have been
modified, in accordance with the supreme law of utility, by means of varia-
tion and survival of the fittest.

xiv FUNDAMENTAL PROBLEMS 435

except when they directly affect the reproductive cells, has
not been proved. On the other hand, as we shall presently
shoAV, there is much reason for believing that such acquired
characters are in their nature non-heritable.

Variation and Selection Overpower the Effects of Use and Disuse.

But there is another objection to this theory arising from
the very nature of the effects produced. In each generation
the effects of use or disuse, or of effort, will certainly be very
small, while of this small effect it is not maintained that the
whole will be always inherited by the next generation. How
small the effect is Ave have no means of determining, except
in the case of disuse, which Mr. Darwin investigated carefully.
He found that in twelve fancy breeds of pigeons, which are
often kept in aviaries, or if free fly but little, the sternum
had been reduced by about one-seventh or one-eighth of its
entire length, and that of the scapula about one-ninth. In
domestic ducks the weight of the wing-bones in proportion to
that of the whole skeleton had decreased about one-tenth.
In domestic rabbits the bones of the legs were found to have
increased in weight in due proportion to the increased weight
of the body, but those of the hind legs were rather less in
proportion to those of the fore legs than in the wild animal,
a difference which may be imputed to their being less used
in rapid motion. The pigeons, therefore, afford the greatest
amount of reduction by disuse — one-seventh of the length of
the sternum. But the pigeon has certainly been domesticated
four or five thousand years ; and if the reduction of the wings
by disuse has only been going on for the last thousand years,
the amount of reduction in each generation would be absolutely
imperceptible, and quite within the limits of the reduction
due to the absence of selection, as already explained. But, as
we have seen in Chapter III, the fortuitous variation of every
part or organ usually amounts to one-tenth, and often to one-
sixth of the average dimensions — that is, the fortuitous varia-
tion in one generation among a limited number of the in-
dividuals of a species is as great as the cumulative effects of
disuse in a thousand generations ! If we assume that the
effects of use or of effort in the individual are equal to the
effects of disuse, or even ten or a hundred times greater, they

436 DARWINISM

will even then not equal, in each generation, the amount of
the fortuitous variations of the same part. If it be urged
that the effects of use would modify all the individuals of a
species, while the fortuitous variations to the amount named
only apply to a portion of them, it may be replied, that that
portion is sufficiently large to afford ample materials for
selection, since it often equals the numbers that can annually
survive ; while the recurrence in each successive generation of
a like amount of variation would render possible such a rapid
adjustment to new conditions that the effects of use or disuse
would be as nothing in comparison. It follows, that even
admitting the modifying effects of the environment, and that
such modifications are inherited, they would yet be entirely
swamped by the greater effects of fortuitous variation, and the
far more rapid cumulative results of the selection of such
variations.

Supposed Action of the Environment in Initiating Variations.

It is, however, urged that the reaction of the environment
initiates variations, which without it would never arise ; such,
for instance, as the origin of horns through the pressures and
irritations caused by butting, or otherwise using the head as a
weapon or for defence. Admitting, for the sake of argument,
that this is so, all the evidence we possess shows that, from the
very first appearance of the rudiment of such an organ, it would
vary to a greater extent than the amount of growth directly
produced by use ; and these variations would be subject to
selection, and would thus modify the organ in ways which use
alone would never bring about. We have seen that this has
been the case with the branching antlers of the stag, which
have been modified by selection, so as to become useful
in other ways than as a mere weapon ; and the same has
almost certainly been the case with the variously curved
and twisted horns of antelopes. In like manner, every con-
ceivable rudiment would, from its first appearance, be subject
to the law of variation and selection, to which, thenceforth,
the direct effect of the environment would be altogether
subordinate.

A very similar mode of reasoning will apply to the other
branch of the subject — the initiation of structures and organs

xiv FUNDAMENTAL PROBLEMS 437

by the action of the fundamental laws of growth. Admitting
that such laws have determined some of the main divisions of
the animal and vegetable kingdom, have originated certain
important organs, and have been the fundamental cause of
certain lines of development, yet at every step of the process
these laws must have acted in entire subordination to the law of
natural selection. No modification thus initiated could have
advanced a single step, unless it were, on the whole, a useful
modification ; while its entire future course would be necessarily
subject to the laws of variation and selection, by which it
would be sometimes checked, sometimes hastened on, sometimes
diverted to one purpose, sometimes to another, according as the
needs of the organism, under the special conditions of its
existence, required such modification. We need not deny that
such laws and influences may have acted in the manner
suggested, but what we do deny is that they could possibly
escape from the ever-present and all-powerful modifying effects
of variation and natural selection.1

Weismann's Theory of Heredity.

Professor August Weismann has put forth a new theory of
heredity founded upon the " continuity of the germ-plasm,"
one of the logical consequences of which is, that acquired
characters of whatever kind are not transmitted from parent to
offspring. As this is a matter of vital importance to the theory
of natural selection, and as, if well founded, it strikes away the
foundations of most of the theories discussed in the present
chapter, a brief outline of Weismann's views must be attempted,

1 In an essay on "The Duration of Life," forming part of the translation
of Dr. Weismann's papers already referred to, the author still further
extends the sphere of natural selection by showing that the average duration
of life in each species has been determined by it. A certain length of life is
essential in order that the species may produce offspring sufficient to ensure
its continuance tinder the most unfavourable conditions ; and it is shown that
the remarkable inequalities of longevity in different species and groups may
be thus accounted for. Yet more, the occurrence of death in the higher
organisms, in place of the continued survival of the unicellular organisms how-
ever much they may increase by subdivision, may be traced to the same great
law of utility for the race and survival of the fittest. The whole essay is of
exceeding interest, and will repay a careful perusal. A similar idea occurred
to the present writer about twenty years back, and was briefly noted down at
the time, but subsequently forgotten.

438 DARWINISM

although it is very difficult to make them intelligible to persons
unfamiliar with the main facts of modern embryology.1

The problem is thus stated by Weismann : " How is it
that in the case of all higher animals and plants a single cell
is able to separate itself from amongst the millions of most
various kinds of which an organism is composed, and by
division and complicated differentiation to reconstruct a new
individual with marvellous likeness, unchanged in many cases
even throughout whole geological periods 1 " Darwin at-
tempted to solve the problem by his theory of "Pangenesis,"
which supposed that every individual cell in the body gave off
gemmules or germs capable of reproducing themselves, and that
portions of these germs of each of the almost infinite number of
cells permeate the whole body and become collected in the
generative cells, and are thus able to reproduce the Avhole
organism. This theory is felt to be so ponderously complex
and difficult that it has met with no general acceptance among
physiologists.

The fact that the germ-cells do reproduce with wonderful
accuracy not only the general characters of the species, but
many of the individual characteristics of the parents or more
remote ancestors, and that this process is continued from
generation to generation, can be accounted for, Weismann
thinks, only on two suppositions which are physiologically
possible. Either the substance of the parent germ-cell, after
passing through a cycle of changes required for the construction
of a new individual, possesses the capability of producing anew
germ-cells identical Avith those from which that individual was
developed, or the new germ-cells arise, as far as their essential
and characteristic substance is concerned, not at (til out of the body
of the individual, but direct from the parent germ-cell. This latter
view Weismann holds to be the correct one, and, on this theory,
heredity depends on the fact that a substance of special mole-
cular composition passes over from one generation to another.
This is the " germ-plasm," the power of which to develop itself
into a perfect organism depends on the extraordinary complica-
tion of its minutest structure. At every new birth a portion

1 The outline here given is derived from two articles in Nature, vol.
xxxiii. p. 154, and vol. xxxiv. p. 629, in which Weismann's papers are summar-
ised and partly translated.

xiv FUNDAMENTAL PROBLEMS 439

of the specific germ-plasm, which the parent egg-cell contains,
is not used up in producing the offspring, but is reserved un-
changed to produce the germ-cells of the following generation.
Thus the germ-cells — so far as regards their essential part the
germ-plasm — are not a product of the body itself, but are
related to one another in the same way as are a series of
generations of unicellular organisms derived from one another
by a continuous course of simple division. Thus the question
of heredity is reduced to one of growth. A minute portion
of the very same germ-plasm from which, first the germ-cell,
and then the whole organism of the parent, were developed,
becomes the starting-point of the growth of the child.

The Cause of Variation.

But if this were all, the offspring would reproduce the
parent exactly, in every detail of form and structure ; and
here we see the importance of sex, for each new germ grows
out of the united germ-plasms of two parents, whence arises a
mingling of their characters in the offspring. This occurs in each
generation ; hence every individual is a complex result repro-
ducing in ever- varying degrees the diverse characteristics of his
two parents, four grandparents, eight great-grandparents, and
other more remote ancestors ; and that ever-present individual
variation arises which furnishes the material for natural selec-
tion to act upon. Diversity of sex becomes, therefore, of primary
importance as the cause of variation. Where asexual genera-
tion prevails, the characteristics of the individual alone are
reproduced, and there are thus no means of effecting the
change of form or structure required by changed conditions of
existence. Under such changed conditions a complex organ-
ism, if only asexually propagated, would become extinct. But
when a complex organism is sexually propagated, there is an
ever-present cause of change which, though slight in any one
generation, is cumulative, and under the influence of selection
is sufficient to keep up the harmony between the organism
and its slowly changing environment.1

1 There are many indications that this explanation of the cause of variation
is the true one. Mr. E. B. Poulton suggests one, in the fact that partheno-
genetic reproduction only occurs in isolated species, not in groups of related
species ; as this shows that parthenogenesis cannot lead to the evolution of

440 DARWINISM

The Non-Heredity of Acquired Characters.

Certain observations on the embryology of the lower
animals are held to afford direct proof of this theory of heredity,
but they are too technical to be made clear to ordinary
readers. A logical result of the theory is the impossibility of
the transmission of acquired characters, since the molecular
structure of the germ-plasm is already determined within the
embryo ; and Weismann holds that there are no facts which
really prove that acquired characters can be inherited, although
their inheritance has, by most writers, been considered so prob-
able as hardly to stand in need of direct proof.

We have already shown, in the earlier part of this chapter,
that many instances of change, imputed to the inheritance of
acquired variations, are really cases of selection ; while the very
fact that use implies usefulness renders it almost impossible to
eliminate the action of selection in a state of nature. As
regards mutilations, it is generally admitted that they are not
hereditary, and there is ample evidence on this point. When
it was the fashion to dock horses' tails, it was not found that
horses were born with short tails ; nor are Chinese women
born with distorted feet ; nor are any of the numerous forms
of racial mutilation in man, which have in some cases been
carried on for hundreds of generations, inherited. Neverthe-
less, a few cases of apparent inheritance of mutilations have
been recorded,1 and these, if trustworthy, are difficulties in the
way of the theory. The undoubted inheritance of disease is
hardly a difficulty, because the predisposition to disease is a
congenital, not an acquired character, and as such would be the
subject of inheritance. The often-quoted case of a disease
induced by mutilation being inherited (Brown-Sequard's
epileptic guinea-pigs) has been discussed by Professor Weis-
mann, and shown to be not conclusive. The mutilation itself
— a section of certain nerves — was never inherited, but

new forms. Again, in parthenogenetic females the complete apparatus for
fertilisation remains unreduced ; but if these varied as do sexually produced
animals, the organs referred to, being unused, would become rudimentary.

Even more important is the significance of the ' ' polar bodies, " as explained
by Weismann in one of his Essays ; since, if his interpretation of them be
correct, variability is a necessary consequence of sexual generation.

1 Darwin's Animals and Plants, vol. ii. pp. 23, 24.

xiv FUNDAMENTAL PROBLEMS 441

the resulting epilepsy, or a general state of weakness, de-
formity, or sores, was sometimes inherited. It is, however,
possible that the mere injury introduced and encouraged the
growth of certain microbes, which, spreading through the
organism, sometimes reached the germ-cells, and thus trans-
mitted a diseased condition to the offspring. Such a transfer-
ence of microbes is believed to occur in syphilis and tuberculosis,
and has been ascertained to occur in the case of the muscardine
silkworm disease.1

The Theory of Instinct.

The theory now briefly outlined cannot be said to be
proved, but it commends itself to many physiologists as being
inherently probable, and as furnishing a good working
hypothesis till displaced by a better. We cannot, therefore,
accept any arguments against the agency of natural selection
which are based upon the opposite and equally unproved
theory that acquired characters are inherited; and as this
applies to the whole school of what may be termed Neo-
Lamarckians, their speculations cease to have any weight.

The same remark applies to the popular theory of instincts
as being inherited habits ; though Darwin gave very little
weight to this, but derived almost all instincts from spontaneous
useful variations which, like other spontaneous variations, are
of course inherited. At first sight it appears as if the acquired
habits of our trained dogs — pointers, retrievers, etc.- — are
certainly inherited ; but this need not be the case, because
there must be some structural or psychical peculiarities, such
as modifications in the attachments of muscles, increased
delicacy of smell or sight, or peculiar likes and dislikes,
which are inherited ; and from these, peculiar habits follow
as a natural consequence, or are easily acquired. Now, as
selection has been constantly at work in improving all our
domestic animals, we have unconsciously modified the structure,
while preserving only those animals which best served our
purpose in their peculiar faculties, instincts, or habits.

1 In his essay on " Heredity," Dr. Weismann discusses many other cases
of sujiposed inheritance of acquired characters, and shows that they can all
be explained in other ways. Shortsightedness among civilised nations, for
example, is due jDartly to the absence of selection and consequent regression
towards a mean, and partly to its individual production by constant reading.

442 DARWINISM

Much, of the mystery of instinct arises from the persistent
refusal to recognise the agency of imitation, memory, observa-
tion, and reason as often forming part of it. Yet there is
ample evidence that such agency must be taken into account.
Both Wilson and.Leroy state that young birds build inferior
nests to old ones, and the latter author observes that the best
nests are made by birds whose young remain longest in the
nest. So, migration is now Avell ascertained to be effected by
means of vision, long nights being made on bright moonlight
nights when the birds fly very high, while on cloudy nights
they fly low, and then often lose their way. Thousands
annually fly out to sea and perish, showing that the instinct
to migrate is imperfect, and is not a good substitute for reason
and observation.

Again, much of the perfection of instinct is due to the
extreme severity of the selection during its development, any
failure involving destruction. The chick which cannot break
the eggshell, the caterpillar that fails to suspend itself properly
or to spin a safe cocoon, the bees that lose their ivay or that
fail to store honey, inevitably perish. So the birds that fail
to feed and protect their young, or the butterflies that lay
their eggs on the wrong food-plant, leave no offspring, and
the race with imperfect instincts perishes. Now, during the
long and very slow course of development of each organism,
this rigid selection at every step of progress has led to the
preservation of every detail of structure, faculty, or habit that
has been necessary for the preservation of the race, and has
thus gradually built up the various instincts which seem so
marvellous to us, but which can yet be shown to be in many
cases still imperfect. Here, as everywhere else in nature, we
find comparative, not absolute perfection, with every gradation
from what is clearly due to imitation or reason up to what
seems to us perfect instinct — that in which a complex action
is performed without any previous experience or instruction.1

1 Weismann explains instinct on similar lines, and gives many interesting
illustrations (see Essays on Heredity). He holds "that all instinct is entirely
due to the operation of natural selection, and has its foundation, not upon
inherited experiences, but upon variations of the germ." Many interesting
and difficult cases of instinct are discussed by Darwin in Chapter VIII of the
Orir/in of Species, which should be read in connection with the above remarks.

Since this chapter was written my attention has been directed to Mr.

FUNDAMENTAL PROBLEMS 443

Concluding Remarks.

Having now passed in review the more important of the
recent objections to, or criticisms of, the theory of natural
selection, we have arrived at the conclusion that in no one
case have the writers in question been able materially to
diminish its importance, or to show that any of the laws or
forces to which they appeal can act otherwise than in strict
subordination to it. The direct action of the environment as
set forth by Mr. Herbert Spencer, Dr. Cope, and Dr. Karl
Semper, even if we admit that its effects on the individual
are transmitted by inheritance, are so small in comparison
with the amount of spontaneous variation of every part of
the organism that they must be quite overshadowed by the
latter. And if such direct action may, in some cases, have
initiated certain organs or outgrowths, these must from their
very first beginnings have been subject to variation and
natural selection, and their further development have been
almost wholly due to these ever-present and powerful causes.

Francis Galton's Theory of Heredity (already referred to at p. 417) which
was published thirteen years ago as an alternative for Darwin's theory of
pangenesis.

Mr. Galton's theory, although it attracted little attention, appears to me
to be substantially the same as that of Professor Weismann. Galton's
"stirp " is Weismann's "germ-plasm." Galton supposes the sexual elements
in the offspring to be directly formed from the residue of the stirp not used
up in the development of the body of the parent — Weismann's " continuity
of the germ-plasm." Galton also draws many of the same conclusions from
his theory. He maintains that characters acquired by the individual as the
result of external influences cannot be inherited, unless such influences act
directly on the reproductive elements — instancing the possible heredity of
alcoholism, because the alcohol permeates the tissues and may reach the
sexual elements. He discusses the supposed heredity of effects produced by
use or disuse, and explains them much in the same manner as does Weismann.
Galton is an anthropologist, and applies the theory, mainly, to explain the
peculiarities of hereditary transmission in man, many of which peculiarities
he discusses and elucidates. Weismann is a biologist, and is mostly concerned
with the application of the theory to explain variation and instinct, and to
the further development of the theory of evolution. He has worked it out
more thoroughly, and has adduced embryological evidence in its support ; but
the views of both writers are substantially the same, and their theories were
arrived at quite independently. The names of Galton and Weismann should
therefore be associated as discoverers of what may be considered (if finally
established) the most important contribution to the evolution theory since the
appearance of the Origin of Species.

444 DARWINISM chap, xiv

The same remark applies to the views of Professor Geddes on
the laws of growth which have determined certain essential
features in the morphology of plants and animals. The
attempt to substitute these laws for those of variation and
natural selection 'has failed in cases where we can apply a
definite test, as in that of the origin of spines on trees and
shrubs ; while the extreme diversity of vegetable structure
and form among the plants of the same country and of the
same natural order, of itself affords a proof of the preponder-
ating influence of variation and natural selection in keeping
the many diverse forms in harmony with the highly complex
and ever-changing environment.

Lastly, we have seen that Professor Weismann's theory of
the continuity of the germ -plasm and the consequent non-
heredity of acquired characters, while in perfect harmony
with all the well-ascertained facts of heredity and development,
adds greatly to the importance of natural selection as the one
invariable and ever-present factor in all organic change, and
that which can alone have produced the temporary fixity
combined with the secular modification of species. While
admitting, as Darwin always admitted, the co-operation of the
fundamental laws of growth and variation, of correlation and
heredity, in determining the direction of lines of variation
or in the initiation of peculiar organs, we find that variation
and natural selection are ever-present agencies, which take
possession, as it were, of every minute change originated
by these fundamental causes, check or favour their further
development, or modify them in countless varied ways
according to the varying needs of the organism. Whatever
other causes have been at work, Natural Selection is supreme,
to an extent which even Darwin himself hesitated to claim
for it. The more we study it the more we are convinced of
its overpowering importance, and the more confidently we
claim, in Darwin's own words, that it " has been the most
important, but not the exclusive, means of modification."

CHAPTER XV

DARWINISM APPLIED TO MAN

General identity of human and animal structure — Rudiments and varia-
tions showing relation of man to other mammals — The embryonic
development of man and other mammalia — Diseases common to man
and the lower animals — The animals most nearly allied to man —
The brains of man and apes— External differences of man and apes —
Summary of the animal characteristics of man — The geological
antiquity of man — The probable birthplace of man — The origin of
the moral and intellectual nature of man — The argument from
continuity — The origin of the mathematical faculty — The origin of
the musical and artistic faculties — Independent proof that these
faculties have not been developed by natural selection — The inter-
pretation of the facts — Concluding remarks.

Our review of modern Darwinism might fitly have terminated
with the preceding chapter ; but the immense interest that
attaches to the origin of the human race, and the amount of
misconception which prevails regarding the essential teachings
of Darwin's theory on this question, as well as regarding my
own special views upon it, induce me to devote a final chapter
to its discussion.

To any one who considers the structure of man's body,
even m the most superficial manner, it must be evident that
it is the body of an animal, differing greatly, it is true, from
the bodies of all other animals, but agreeing with them in all
essential features. The bony structure of man classes him as
a vertebrate ; the mode of suckling his young classes him as
a mammal ; his blood, his muscles, and his nerves, the structure
of his heart with its veins and arteries, his lungs and his whole
respiratory and circulatory systems, all closely correspond to
those of other mammals, and are often almost identical with

4 46 DARWINISM

them. He possesses the same number of limbs terminating
in the same number of digits as belong fundamentally to the
mammalian class. His senses are identical with theirs, and
his organs of sense are the same in number and occupy the
same relative position. Every detail of structure which is
common to the mammalia as a class is found also in man,
while he only differs from them in such ways and degrees as
the various species or groups of mammals differ from each
other. If, then, we have good reason to believe that every
existing group of mammalia has descended from some common
ancestral form — as we saw to be so completely demonstrated
in the case of the horse tribe, — and that each family, each
order, and even the whole class must similarly have de-
scended from some much more ancient and more generalised
type, it would be in the highest degree improbable — so
improbable as to be almost inconceivable— that man, agreeing
with them so closely in every detail of his structure, should
have had some quite distinct mode of origin. Let us, then,
see what other evidence bears upon the question, and whether
it is sufficient to convert the probability of his animal origin
into a practical certainty.

Rudiments and Variations as Indicating the Relation of Man to
other Mammals.

All the higher animals present rudiments of organs which,
though useless to them, are useful in some allied group, and
are believed to have descended from a common ancestor in
which they were useful. Thus there are in ruminants rudi-
ments of incisor teeth which, in some species, never cut through
the gums ; many lizards have external rudimentary legs ;
while many birds, as the Apteryx, have quite rudimentary
wings. Now man possesses similar rudiments, sometimes
constantly, sometimes only occasionally present, which serve
intimately to connect his bodily structure with that of the
lower animals. Many animals, for example, have a special
muscle for moving or twitching the skin. In man there are
remnants of this in certain parts of the body, especially in
the forehead, enabling us to raise our eyebrows ; but some
persons have it in other parts. A few persons are able to move
the whole scalp so as to throw off any object placed on the head,

xv DARWINISM APPLIED TO MAN 447

and this property has been proved, in one case, to be inherited.
In the outer fold of the ear there is sometimes a projecting point,
corresponding in position to the pointed ear of many animals,
and believed to be a rudiment of it. In the alimentary canal
there is a rudiment — the vermiform appendage of the caecum —
which is not only useless, but is sometimes a cause of disease
and death in man ; yet in many vegetable feeding animals it
is very long, and even in the orang-utan it is of considerable
length and convoluted. So, man possesses rudimentary bones
of a tail concealed beneath the skin, and, in some rare cases,
this forms a minute external tail.

The variability of every part of man's structure is very
great, and many of these variations tend to approximate
towards the structure of other animals. The courses of the
arteries are eminently variable, so that for surgical purposes
it has be~en necessary to determine the probable proportion of
each variation. The muscles are so variable that in fifty cases
the muscles of the foot were found to be not strictly alike in
any two, and in some the deviations were considerable ; while
in thirty-six subjects Mr. J. Wood observed no fewer than 558
muscular variations. The same author states that in a single
male subject there were no fewer than seven muscular varia-
tions, all of which plainly represented muscles proper to various
kinds of apes. The muscles of the hands and arms — parts
which are so eminently characteristic of man — are extremely
liable to vary, so as to resemble the corresponding muscles of
the lower animals. That such variations are due to reversion
to a former state of existence Mr. Darwin thinks highly prob-
able, and he adds : " It is quite incredible that a man should,
through mere accident, abnormally resemble certain apes in
no less than seven of his muscles, if there had been no genetic
connection between them. On the other hand, if man is
descended from some ape-like creature, no valid reason can be
assigned why certain muscles should not suddenly reappear
after an interval of many thousand generations, in the same
manner as, with horses, asses, and mules, dark coloured
stripes suddenly reappear on the legs and shoulders, after
an interval of hundreds, or more probably of thousands of
generations." x

1 Descent of Man, pp. 41-43 ; also pp. 13-15.

448 DARWINISM

The Embryonic Development of Man and other Mammalia.

The progressive development of any vertebrate from the
ovum or minute embryonic egg affords one of the most
marvellous chapters in Natural History. We see the con-
tents of the ovum undergoing numerous definite changes,
its interior dividing and subdividing till it consists of a
mass of cells, then a groove appears marking out the median
line or vertebral column of the future animal, and there-
after are slowly developed the various essential organs of
the body. After describing in some detail what takes place
in the case of the ovum of the dog, Professor Huxley
continues: "The history of the development of any other
vertebrate animal, lizard, snake, frog, or fish tells the same
story. There is always to begin with, an egg having the
same essential structure as that of the dog ; the yelk of
that egg undergoes division or segmentation, as it is called,
the ultimate products of that segmentation constitute the
building materials for the body of the young animal ; and
this is built up round a primitive groove, in the floor of which
a notochord is developed. Furthermore, there is a period in
which the young of all these animals resemble one another,
not merely in outward form, but in all essentials of structure,
so closely, that the differences between them are inconsider-
able, while in their subsequent course they diverge more and
more widely from one another. And it is a general law that
the more closely any animals resemble one another in adult
structure, the larger and the more intimately do their embryos
resemble one another ; so that, for example, the embryos of a
snake and of a lizard remain like one another longer than do
those of a snake and a bird ; and the embryos of a dog and
of a cat remain like one another for a far longer period than
do those of a dog and a bird, or of a dog and an opossum, or
even than those of a dog and a monkey."1

We thus see that the study of development affords a test
of affinity in animals that are externally very much unlike
each other ; and we naturally ask how this applies to man.
Is he developed in a different way from other mammals, as
we should certainly expect if he has had a distinct and
1 Man's Place in Nature, p. 64.

xv DARWINISM APPLIED TO MAN 449

altogether different origin ? " The reply," says Professor
Huxley, " is not doubtful for a moment. Without question,
the mode of origin and the early stages of the development of
man are identical with those of the animals immediately
below him in the scale." And again he tells us : " It is very
long before the body of the young human being can be readily
discriminated from that of the young puppy ; but at a
tolerably early period the two become distinguishable by the
different forms of their adjuncts, the yelk -sac and the al-
lantois ; " and after describing these differences he continues :
" But exactly in those respects in which the developing man
differs from the dog, he resembles the ape. ... So that it is
only quite in the latter stages of development that the young
human being presents marked differences from the young ape,
while the latter departs as much from the dog in its develop-
ment as the man does. Startling as this last assertion may
appear to be, it is demonstrably true, and it alone appears
to me sufficient to place beyond all doubt the structural unity
of man with the rest of the animal world, and more par-
ticularly and closely with the apes."1

A few of the curious details in which man passes through
stages common to the lower animals may be mentioned. At
one stage the os coccyx projects like a true tail, extending
considerably beyond the rudimentary legs. In the seventh
month the convolutions of the brain resemble those of an
adult baboon. The great toe, so characteristic of man,
forming the fulcrum which most assists him in standing erect,
in an early stage of the embryo is much shorter than the
other toes, and instead of being parallel with them, projects
at an angle from the side of the foot, thus corresponding with
its permanent condition in the quadrumana. Numerous other
examples might be quoted, all illustrating the same general
law.

Diseases Common to Man and the Lower Animals.

Though the fact is so well known, it is certainly one of
profound significance that many animal diseases can be com-
municated to man, since it shows similarity, if not identity, in

1 Man's Place in Nature, p. 67. See Figs, of Embryos of Man and Dog
in Darwin's Descent of Man, p. 10.

2 G

450 DARWINISM

the minute structure of the tissues, the nature of the blood,
the nerves, and the brain. Such diseases as hydrophobia,
variola, the glanders, cholera, herpes, etc., can be transmitted
from animals to man or the reverse ; while monkeys are liable
to many of the same non-contagious diseases as we are.
Rengger, who carefully observed the common monkey (Cebus
Azarse). in Paraguay, found it liable to catarrh, with the usual
symptoms, terminating sometimes in consumption. These
monkeys also suffered from apoplexy, inflammation of the
bowels, and cataract in the eye. Medicines produced the
same effect upon them as upon us. Many kinds of monkeys
have a strong taste for tea, coffee, spirits, and even tobacco.
These facts show the similarity of the nerves of taste in
monkeys and in ourselves, and that their whole nervous
system is affected in a similar way. Even the parasites, both
external and internal, that affect man are not altogether
peculiar to him, but belong to the same families or genera as
those which infest animals, and in one case, scabies, even the
same species.1 These curious facts seem quite inconsistent
with the idea that man's bodily structure and nature are
altogether distinct from those of animals, and have had a
different origin ; while the facts are just what we should
expect if he has been produced by descent with modification
from some common ancestor.

The Animals most nearly Allied to Man.

By universal consent we see in the monkey tribe a
caricature of humanity. Their faces, their hands, their
actions and expressions present ludicrous resemblances to our
own. But there is one group of this great tribe in which this
resemblance is greatest, and they have hence been called the
anthropoid or man -like apes. These are few in number, and
inhabit only the equatorial regions of Africa and Asia, countries
where the climate is most uniform, the forests densest, and
the supply of fruit abundant throughout the year. These
animals are now comparatively well known, consisting of the
orang-utan of Borneo and Sumatra, the chimpanzee and the
gorilla of West Africa, and the group of gibbons or long-armed
apes, consisting of many species and inhabiting South-Eastern
1 The Descent of Man, pp. 7, 8,

xv DARWINISM APPLIED TO MAN 451

Asia and the larger Malay Islands. These last are far less like
man than the other three, one or other of which has at
various times been claimed to be the most man -like of the
apes and our nearest relations in the animal kingdom. The
question of the degree of resemblance of these animals to
ourselves is one of great interest, leading, as it does, to some
important conclusions as to our origin and geological antiquity,
and we will therefore briefly consider it.

If we compare the skeletons of the orang or chimpanzee
with that of man, we find them to be a kind of distorted
copy, every bone corresponding (with very few exceptions),
but altered somewhat in size, proportions, and position. So
great is this resemblance that it led Professor Owen to
remark : " I cannot shut my eyes to the significance of that
all-pervading similitude of structure — every tooth, every bone,
strictly homologous — which makes the determination of the
difference between Homo and Pithecus the anatomist's diffi-
culty."

The actual differences in the skeletons of these apes and
that of man — that is, differences dependent on the presence
or absence of certain bones, and not on their form or position
— have been enumerated by Mr. Mivart as follows : — (1) In
the breast-bone consisting of but two bones, man agrees with
the gibbons ; the chimpanzee and gorilla having this part
consisting of seven bones in a single series, while in the
orang they are arranged in a double series of ten bones. (2)
The normal number of the ribs in the orang and some gibbons
is twelve pairs, as in man, while in the chimpanzee and gorilla
there are thirteen pairs. (3) The orang and the gibbons also
agree with man in having five lumbar vertebrae, while in the
gorilla and the chimpanzee there are but four, and sometimes
only three. (4) The gorilla and chimpanzee agree with man
in having eight small bones in the wrist, while the orang and
the gibbons, as well as all other monkeys, have nine.1

The differences in the form, size, and attachments of the
various bones, muscles, and other organs of these apes and

1 Man and Apes. By St. George Mivart, F.R.S., 1873. It is an
interesting fact (for which I am indebted to Mr. E. B. Poulton) that the
human embryo possesses the extra rib and wrist-bone referred to above in
(2) and (4) as occurring in some of the apes.

452 DARWINISM

man are very numerous and exceedingly complex, sometimes
one species, sometimes another agreeing most nearly with
ourselves, thus presenting a tangled web of affinities which it
is very difficult to unravel. Estimated by the skeleton alone,
the chimpanzee and gorilla seem nearer to man than the
orang, which last is also inferior as presenting certain aberra-
tions in" the muscles. In the form of the ear the gorilla is
more human than any other ape, while in the tongue the
orang is the more man -like. In the stomach and liver the
gibbons approach nearest to man, then come the orang and
chimpanzee, while the gorilla has a degraded liver more
resembling that of the lower monkeys and baboons.

The Brains of Man and Apes.

We come now to that part of his organisation in which
man is so much higher than all the lower animals — the brain ;
and here, Mr. Mivart informs us, the orang stands highest
in rank. The height of the orang's cerebrum in front is
greater in proportion than in either the chimpanzee or the
gorilla. " On comparing the brain of man with the brains of
the orang, chimpanzee, and baboon, we find a successive
decrease in the frontal lobe, and a successive and very great
increase in the relative size of the occipital lobe. Con-
comitantly with this increase and decrease, certain folds of
brain substance, called ' bridging convolutions,' which in man
are conspicuously interposed between the parietal and
occipital lobes, seem as utterly to disappear in the chim-
panzee, as they do in the baboon. In the orang, however,
though much reduced, they are still to be distinguished. . . .
The actual and absolute mass of the brain is, however, slightly
greater in the chimpanzee than in the orang, as is the relative
vertical extent of the middle part of the cerebrum, although,
as already stated, the frontal portion is higher in the orang ;
while, according to M. Gratiolet, the gorilla is not only
inferior to the orang in cerebral development, but even to his
smaller African congener, the chimpanzee." 1

On the whole, then, we find that no one of the great apes
can be positively asserted to be nearest to man in structure.
Each of them approaches him in certain characteristics, while
1 Man and Apes, pp. 138, 144.

xv DARWINISM APPLIED TO MAN 453

in. others it is widely removed, giving the idea, so consonant
with the theory of evolution as developed by Darwin, that all
are derived from a common ancestor, from which the existing
anthropoid apes as well as man have diverged. When,
however, we turn from the details of anatomy to peculiarities
of external form and motions, we find that, in a variety of
characters, all these apes resemble each other and differ from
man, so that we may fairly say that, while they have diverged
somewhat from each other, they have diverged much more
widely from ourselves. Let us briefly enumerate some of
these differences.

External Differences of Man and Apes.

All apes have large canine teeth, while in man these are
no longer than the adjacent incisors or premolars, the whole
forming a perfectly even series. In apes the arms are pro-
portionately much longer than in man, while the thighs are
much shorter. No ape stands really erect, a posture which
is natural in man. The thumb is proportionately larger in
man, and more perfectly opposable than in that of any ape.
The foot of man differs largely from that of all apes, in the
horizontal sole, the projecting heel, the short toes, and the
powerful great toe firmly attached parallel to the other toes ;
all perfectly adapted for maintaining the erect posture, and
for free motion without any aid from the arms or hands. In
apes the foot is formed almost exactly like our hand, with
a large thumb-like great toe quite free from the other toes,
and so articulated as to be opposable to them ; forming with
the long finger-like toes a perfect grasping hand. The sole
cannot be placed horizontally on the ground ; but when
standing on a level surface the animal rests on the outer edge
of the foot with the finger and thumb-like toes partly closed,
while the hands are placed on the ground resting on the
knuckles. The illustration on the next page (Fig. 37) shows,
fairly well, the peculiarities of the hands and feet of the chim-
panzee, and their marked differences, both in form and use,
from those of man.

The four limbs, with the peculiarly formed feet and hands,
are those of arboreal animals which only occasionally and
awkwardly move on level ground. The arms are used in pro-

454

DARWINISM

gression equally with the feet, and the hands are only adapted
for uses similar to those of our hands when the animal is at rest,
and then but clumsily. Lastly, the apes are all hairy animals,
like the majority of other mammals, man alone having a
smooth and almost naked skin. These numerous and striking
differences, even more than those of the skeleton and internal

Fig. 37.— Chimpanzee (Troglodytes niger).

anatomy, point to an enormously remote epoch when the race
that was ultimately to develop into man diverged from that
other stock which continued the animal type and ultimately
produced the existing varieties of anthropoid apes.

Summary of the Animal Characteristics of 3 fan.

The facts now very briefly summarised amount almost to
a demonstration that man, in his bodily structure, has been
derived from the lower animals, of which he is the culminating
development. In his possession of rudimentary structures

xv DARWINISM APPLIED TO MAN 455

which are functional in some of the mammalia ; in the
numerous variations of his muscles and other organs agreeing
with characters which are constant in some apes ; in his
embryonic development, absolutely identical in character with
that of mammalia in general, and closely resembling in its details
that of the higher quadrumana ; in the diseases which he has
in common with other mammalia ; and in the wonderful
approximation of his skeleton to those of one or other of the
anthropoid apes, we have an amount of evidence in this
direction which it seems impossible to explain away. And
this evidence will appear more forcible if we consider for
a moment what the rejection of it implies. For the only
alternative supposition is, that man has been specially created —
that is to say, has been produced in some quite different way
from other animals and altogether independently of them.
But in that case the rudimentary structures, the animal-like
variations, the identical course of development, and all the other
animal characteristics he possesses are deceptive, and inevitably
lead us, .as thinking beings making use of the reason which is
our noblest and most distinctive feature, into gross error.

We cannot believe, however, that a careful study of the
facts of nature leads to conclusions directly opposed to the
truth ; and, as we seek in vain, in our physical structure and
the course of its development, for any indication of an origin
independent of the rest of the animal World, we are compelled
to reject the idea of "special creation "for man, as being
entirely unsupported by facts as well as in the highest degree
improbable.

The Geological Antiquity of Man.

The evidence we now possess of the exact nature of the
resemblance of man to the various species of anthropoid apes,
shows us that he has little special affinity for any one rather
than another species, while he differs from them all in several
important characters in which they agree with each other.
The conclusion to be drawn from these facts is, that his points
of affinity connect him with the whole group, while his special
peculiarities equally separate him from the whole group, and
that he must, therefore, have diverged from the common
ancestral form before the existing types of anthropoid apes

456 DARWINISM

had diverged from each other. Now, this divergence almost
certainly took place as early as the Miocene period, because in
the Upper Miocene deposits of Western Europe remains of two
species of ape have been found allied to the gibbons, one of
them, Dryopithecus, nearly as large as a man, and believed by
M. Lartet to have approached man in its dentition more than
the existing apes. We seem hardly, therefore, to have reached,
in the Upper Miocene, the epoch of the common ancestor of
man and the anthropoids.

The evidence of the antiquity of man himself is also scanty,
and takes us but very little way back into the past. We
have clear proof of his existence in Europe in the latter stages
of the glacial epoch, with many indications of his presence in
interglacial or even pre-glacial times ; while both the actual
remains and the works of man found in the auriferous gravels
of California deep under lava-flows of Pliocene age, show that
he existed in the New World at least as early as in the
Old.1 These earliest remains of man have been received
with doubt, and even with ridicule, as if there were some
extreme improbability in them. But, in point of fact,
the wonder is that human remains have not been found
more frequently in pre-glacial deposits. Referring to the
most ancient fossil remains found in Europe — the Engis
and Neanderthal crania, — Professor Huxley makes the follow-
ing weighty remark : "In conclusion, I may say, that the
fossil remains of Man hitherto discovered do not seem to me
to take us appreciably nearer to that lower pithecoid form, by
the modification of which he has, probably, become what he
is." The Calif ornian remains and works of art, above referred
to, give no indication of a specially low form of man ; and it
remains an unsolved problem why no traces of the long line
of man's ancestors, back to the remote period when he first
branched off from the pithecoid type, have yet been discovered.

It has been objected by some writers — notably by Professor
Boyd Dawkins — that man did not probably exist in Pliocene
times, because almost all the known mammalia of that epoch
are distinct species from those now living on the earth,
and that the same changes of the environment which led to

1 For a sketch of the evidence of Man's Antiquity in America, see The
Nineteenth Century for November 1887.

xv DARWINISM APPLIED TO MAN 457

the modification of other mammalian species would also have
led to a change in man. But this argument overlooks the
fact that man differs essentially from all other mammals in
this respect, that whereas any important adaptation to new
conditions can be effected in them only by a change in bodily
structure, man is able to adapt himself to much greater
changes of conditions by a mental development leading him
to the use of fire, of tools, of clothing, of improved dwellings, of
nets and snares, and of agriculture. By the help of these,
without any change whatever in his bodily structure, he has
been able to spread over and occupy the whole earth ; to
dwell securely in forest, plain, or mountain ; to inhabit alike
the burning desert or the arctic wastes ; to cope with every
kind of wild beast, and to provide himself with food in
districts where, as an animal trusting to nature's unaided
productions, he would have starved.1

It follows, therefore, that from the time when the ancestral
man first walked erect, with hands freed from any active part
in locomotion, and when his brain-power became sufficient to
cause him to use his hands in making weapons and tools,
houses and clothing, to use fire for cooking, and to plant seeds
or roots to supply himself with stores of food, the power of
natural selection would cease to act in producing modifications
of his body, but would continuously advance his mind through
the development of its organ, the brain. Hence man may
have become truly man — the species, Homo sapiens — even
in the Miocene period ; and while all other mammals were
becoming modified from age to age under the influence of ever-
changing physical and biological conditions, he would be
advancing mainly in intelligence, but perhaps also in stature,
and by that advance alone would be able to maintain himself
as the master of all other animals and as the most widespread
occupier of the earth. It is quite in accordance with this view
that we find the most pronounced distinction between man
and the anthropoid apes in the size and complexity of his
brain. Thus, Professor Huxley tells us that "it may be
doubted whether a healthy human adult brain ever weighed

1 This subject was first discussed in an article in the Anthropological
Review, May 1864, and republished in my Contributions to Natural Selection,
chap, ix, in 1870.

458 DARWINISM

less than 31 or 32 ounces, or that the heaviest gorilla
brain has exceeded 20 ounces," although " a full-grown
gorilla is probably pretty nearly twice as heavy as a Bosjes
man, or as many an European woman."1 The average
human brain, however, weighs 48 or 49 ounces, and if we take
the average ape brain at only 2 ounces less than the very
largest gorilla's brain, or 18 ounces, we shall see better the
enormous increase which has taken place in the brain of man
since the time when he branched off from the apes ; and this
increase will be still greater if we consider that the brains of
apes, like those of all other mammals, have also increased
from earlier to later geological times.

If these various considerations are taken into account, we
must conclude that the essential features of man's structure
as compared with that of apes — his erect posture and free
hands — were acquired at a comparatively early period, and
were, in fact, the characteristics Avhich gave him his superiority
over other mammals, and started him on the line of develop-
ment which has led to his conquest of the world. But during
this long and steady development of brain and intellect, man-
kind must have continuously increased in numbers and in
the area which they occupied — they must have formed what
Darwin terms a " dominant race." For had they been few in
numbers and confined to a limited area, they could hardly
have successfully struggled against the numerous fierce
carnivora of that period, and against those adverse influences
which led to the extinction of so many more powerful
animals. A large population spread over an extensive area
is also needed to supply an adequate number of brain varia-
tions for man's progressive improvement. But this large
population and long-continued development in a single line
of advance renders it the more difficult to account for
the complete absence of human or pre-human remains in
all those deposits which have furnished, in such rich
abundance, the remains of other land animals. It is true
that the remains of apes are also very rare, and we may
well suppose that the superior intelligence of man led him to
avoid that extensive destruction by flood or in morass which
seems to have often overwhelmed other animals. Yet, when
1 Man's Place in Nature, p. 102.

xv DARWINISM APPLIED TO MAN 459

we consider that, even in our own day, men are not unfre-
quently overwhelmed by volcanic eruptions, as in Java and
Japan, or carried away in vast numbers by floods, as in Bengal
and China, it seems impossible but that ample remains of
Miocene and Pliocene man do exist buried in the most recent
layers of the earth's crust, and that more extended research
or some fortunate discovery will some day bring them to
light.

The Probable Birthplace of Man.

It has usually been considered that the ancestral form of
man originated in the tropics, where vegetation is most
abundant and the climate most equable. But there are some
important objections to this view. The anthropoid apes, as
well as most of the monkey tribe, are essentially arboreal in
their structure, whereas the great distinctive character of man
is his special adaptation to terrestrial locomotion. We can
hardly suppose, therefore, that he originated in a forest region,
where fruits to be obtained by climbing are the chief vegetable
food. It is more probable that he began his existence on the
open plains or high plateaux of the temperate or sub-tropical
zone, where the seeds of indigenous cereals and numerous
herbivora, rodents, and game-birds, with fishes and molluscs in
the lakes, rivers, and seas supplied him with an abundance of
varied food. In such a region he would develop skill as a
hunter, trapper, or fisherman, and later as a herdsman and
cultivator, — a succession of which we find indications in the
palaeolithic and neolithic races of Europe.

In seeking to determine the particular areas in which his
earliest traces are likely to be found, we are restricted to
some portion of the Eastern hemisphere, where alone the
anthropoid apes exist, or have apparently ever existed.

There is good reason to believe, also, that Africa must be
excluded, because it is known to have been separated from
the northern continent in early tertiary times, and to have
acquired its existing fauna of the higher mammalia by a
later union with that continent after the separation from it of
Madagascar, an island which has preserved for us a sample, as
it were, of the early African mammalian fauna, from which
not only the anthropoid apes, but all the higher quadrumana

460 DARWINISM chap.

are absent.1 There remains only the great Euro- Asiatic con-
tinent ; and its enormous plateaux, extending from Persia
right across Tibet and Siberia to Manchuria, afford an area,
some part or other of which probably offered suitable con-
ditions, in late Miocene or early Pliocene times, for the develop-
ment of ancestral man.

It is in this area that we still find that type of mankind —
the Mongolian — which retains a colour of the skin midway
between the black or brown-black of the negro, and the ruddy
or olive-white of the Caucasian types, a colour which still
prevails over all Northern Asia, over the American continents,
and over much of Polynesia. From this primary tint arose,
under the influence of varied conditions, and probably in
correlation with constitutional changes adapted to peculiar
climates, the varied tints which still exist among mankind. If
the reasoning by which this conclusion is reached be sound, and
all the earlier stages of man's development from an animal
form occurred in the area now indicated, we can better under-
stand how it is that we have as yet met with no traces of the
missing links, or even of man's existence during late tertiary
times, because no part of the world is so entirely unexplored
by the geologist as this very region. The area in question is
sufficiently extensive and varied to admit of primeval man
having attained to a considerable population, and having
developed his full human characteristics, both physical and
mental, before there was any need for him to migrate beyond
its limits. One of his earliest important migrations was
probably into Africa, where, spreading westward, he became
modified in colour and hair in correlation Avith physiological
changes adapting him to the climate of the equatorial low-
lands. Spreading north-westward into Europe the moist and
cool climate led to a modification of an opposite character, and
thus may have arisen the three great human types which still
exist. Somewhat later, probably, he spread eastward into
North -West America and soon scattered himself over the
whole continent ; and all this may well have occurred in
early or middle Pliocene times. Thereafter, at very long
intervals, successive waves of migration carried him into every

^ 1 For a full discussion of this question, see the author's Geographical
Distribution of Animals, vol. i. p. 285.

xv DARWINISM APPLIED TO MAN 461

part of the habitable world, and by conquest and intermixture
led ultimately to that puzzling gradation of types which the
ethnologist in vain seeks to unravel.

The Origin of the Moral and Intellectual Nature of Man.

From the foregoing discussion it will be seen that I fully
accept Mr. Darwin's conclusion as to the essential identity of
man's bodily structure with that of the higher mammalia, and
his descent from some ancestral form common to man and
the anthropoid apes. The evidence of such descent appears
to me to be overwhelming and conclusive. Again, as to the
cause and method of such descent and modification, we may
admit, at all events provisionally, that the laws of variation
and natural selection, acting through the struggle for existence
and the continual need of more perfect adaptation to the
physical and biological environments, may have brought about,
first that perfection of bodily structure in which he is so far
above all other animals, and in co-ordination with it the
larger and more developed brain, by means of which he has
been able to utilise that structure in the more and more
complete subjection of the whole animal and vegetable king-
doms to his service.

But this is only the beginning of Mr. Darwin's work, since
he goes on to discuss the moral nature and mental faculties of
man, and derives these too by gradual modification and de-
velopment from the lower animals. Although, perhaps, nowhere
distinctly formulated, his whole argument tends to the con-
clusion that man's entire nature and all his faculties, whether
moral, intellectual, or spiritual, have been derived from their
rudiments in the lower animals, in the same manner and by
the action of the same general laws as his physical structure
has been derived. As this conclusion appears to me not to be
supported by adequate evidence, and to be directly opposed to
many well-ascertained facts, I propose to devote a brief space
to its discussion.

The Argument from Continuity.

Mr. Darwin's mode of argument consists in showing that
the rudiments of most, if not of all, the mental and moral
faculties of man can be detected in some animals. The

462 DARWINISM

manifestations of intelligence, amounting in some cases to
distinct acts of reasoning, in many animals, are adduced as
exhibiting in a much less degree the intelligence and reason
of man. Instances of curiosity, imitation, attention, wonder,
and memory are given ; while examples are also adduced
which may be interpreted as proving that animals exhibit
kindness to their fellows, or manifest pride, contempt, and
shame. Some are said to have the rudiments of lansoiasre,
because they utter several different sounds, each of which has
a definite meaning to their fellows or to their young ; others
the rudiments of arithmetic, because they seem to count and
remember up to three, four, or even five. A sense of beauty
is imputed to them on account of their own bright colours or
the use of coloured objects in their nests ; while dogs, cats,
and horses are said to have imagination, because they appear
to be disturbed by dreams. Even some distant approach to
the rudiments of religion is said to be found in the deep love
and complete submission of a dog to his master.1

Turning from animals to man, it is shown that in the
lowest savages many of these faculties are very little advanced
from the condition in which they appear in the higher animals ;
while others, although fairly well exhibited, are yet greatly
inferior to the point of development they have reached in
civilised races. In particular, the moral sense is said to have
been developed from the social instincts of savages, and to
depend mainly on the enduring discomfort produced by any
action which excites the general disapproval of the tribe.
Thus, every act of an individual which is believed to be
contrary to the interests of the tribe, excites its unvarying
disapprobation and is held to be immoral ; while every act, on
the other hand, which is, as a rule, beneficial to the tribe, is
warmly and constantly approved, and is thus considered to
be right or moral. From the mental struggle, when an act
that would benefit self is injurious to the tribe, there arises
conscience ; and thus the social instincts are the foundation of
the moral sense and of the fundamental principles of morality.2

The question of the origin and nature of the moral sense
and of conscience is far too vast and complex to be discussed

1 For a full discussion of all these points, see Descent of Ma?i, cliap. iii.

2 Descent of Man, chap. iv.

xv DARWINISM APPLIED TO MAN 463

here, arid a reference to it has been introduced only to complete
the sketch of Mr. Darwin's view of the continuity and gradual
development of all human faculties frorii the lower animals up
to savages, and from savage up to civilised man. The point
to which I wish specially to call attention is, that to prove
continuity and the progressive development of the intellectual
and moral faculties from animals to man, is not the same as
proving that these faculties have been developed by natural
selection; and this last is what Mr. Darwin has hardly
attempted, although to support his theory it was absolutely
essential to prove it. Because man's physical structure has been
developed from an animal form by natural selection, it does not
necessarily follow that his mental nature, even though developed
•pari passu with it, has been developed by the same causes only.
To illustrate by a physical analogy. Upheaval and depres-
sion of land, combined with sub-aerial denudation by wind
and frost, rain and rivers, and marine denudation on coast-
lines, were long thought to account for all the modelling of
the earth's surface not directly due to volcanic action ; and
in the early editions of Lyell's Principles of Geology these
are the sole causes appealed to. But when the action of
glaciers was studied and the recent occurrence of a glacial epoch
demonstrated as a fact, many phenomena — such as moraines
and other gravel deposits, boulder - clay, erratic boulders,
grooved and rounded rocks, and Alpine lake basins — were seen
to be due to this altogether distinct cause. There was no breach
of continuity, no sudden catastrophe ; the cold period came
on and passed away in the most gradual manner, and its effects
often passed insensibly into those produced by denudation or
upheaval ; yet none the less a new agency appeared at a
definite time, and new effects were produced which, though
continuous with preceding effects, were not due to the same
causes. It is not, therefore, to be assumed, without proof
or against independent evidence, that the later stages of an
apparently continuous development are necessarily due to the
same causes only as the earlier stages. Applying this argu-
ment to the case of man's intellectual and moral nature, I
propose to show that certain definite portions of it could not
have been developed by variation and natural selection alone,
and that, therefore, some other influence, law, or agency is

464 DARWINISM

required to account for them. If this can be clearly shown
for any one or more of the special faculties of intellectual man,
we shall be justified in assuming that the same unknown cause
or power may have had a much wider influence, and may
have profoundly influenced the whole course of his develop-
ment.

The Origin of the Mathematical Faculty.

We have ample evidence that, in all the lower races of man,
what may be termed the mathematical faculty is, either absent,
or, if present, quite unexercised. The Bushmen and the
Brazilian Wood-Indians are said not to count beyond two.
Many Australian tribes only have words for one and two,
which are combined to make three, four, five, or six, beyond
which they do not count. The Damaras of South Africa
only count to three ; and Mr. Galton gives a curious descrip-
tion of how one of them was hopelessly puzzled when he had
sold two sheep for two sticks of tobacco each, and received
four sticks in payment. He could only find out that he was
correctly paid by taking two sticks and then giving one sheep,
then receiving two sticks more and giving the other sheep.
Even the comparatively intellectual Zulus can only count up
to ten by using the hands and fingers. The Ahts of North- West
America count in nearly the same manner, and most of the
tribes of South America are no further advanced.1 The
Kaffirs have great herds of cattle, and if one is lost they miss
it immediately, but this is not by counting, but by noticing
the absence of one they know ; just as in a large family or a
school a boy is missed without going through the process of
counting. Somewhat higher races, as the Esquimaux, can
count up to twenty by using the hands and the feet ; and
other races get even further than this by saying " one man "
for twenty, " two men " for forty, and so on, equivalent to our
rural mode of reckoning by scores. From the fact that so
many of the existing savage races can only count to four or
five, Sir John Lubbock thinks it improbable that our earliest
ancestors could have counted as high as ten.2

1 Lubbock's Origin of Civilisation, fourth edition, pp. 434-440 ; Tylor's
Primitive Culture, chap. vii.

2 It has been recently stated that some of these facts are erroneous, and
that some Australians can keep accurate reckoning up to 100, or more, when

xv DARWINISM APPLIED TO MAN 465

When we turn to the more civilised races, we find the
use of numbers and the art of counting greatly extended.
Even the Tongas of the South Sea islands are said to have
been able to count as high as 100,000. But mere count-
ing does not imply either the possession or the use of any-
thing that can be really called the mathematical faculty, the
exercise of which in any broad sense has only been possible
since the introduction of the decimal notation. The Greeks,
the Romans, the Egyptians, the Jews, and the Chinese had
all such cumbrous systems, that anything like a science of
arithmetic, beyond very simple operations, was impossible ;
and the Roman system, by which the year 1888 would be written
MDCCCLXXXVIII, was that in common use in Europe down
to the fourteenth or fifteenth centuries, and even much later in
some places. Algebra, which was invented by the Hindoos, from
whom also came the decimal notation, was not introduced into
Europe till the thirteenth century, although the Greeks had some
acquaintance with it; and it reached Western Europe from Italy
only in the sixteenth century.1 It was, no doubt, owing to the
absence of a sound system of numeration that the mathematical
talent of the Greeks was directed chiefly to geometry, in which
science Euclid, Archimedes, and others made such brilliant dis-
coveries. It is, however, during the last three centuries only that
the civilised world appears to have become conscious of the
possession of a marvellous faculty which, when supplied with
the necessary tools in the decimal notation, the elements of
algebra and geometry, and the power of rapidly communicating
discoveries and ideas by the art of printing, has developed to
an extent, the full grandeur of which can be appreciated only
by those who have devoted some time (even if unsuccessfully)
to the study.

The facts now set forth as to the almost total absence of
mathematical faculty in savages and its wonderful development
in quite recent times, are exceedingly suggestive, and in regard

required. But this does not alter the general fact that many low races,
including the Australians, have no words for high numbers and never require
to use them. If they are now, with a little practice, able to count much
higher, this indicates the possession of a faculty which could not have been
developed under the law of utility only, since the absence of words for such
high numbers shows that they were neither used nor required.
1 Article Arithmetic in Eng. Cyc. of Arts and Sciences.

2 H

466 DARWINISM

to them we are limited to two possible theories. Either pre-
historic and savage man did not possess this faculty at all
(or only in its merest rudiments) ; or they did possess it, but
had neither the means nor the incitements for its exercise.
In the former case we have to ask by what means has this
faculty been so rapidly developed in all civilised races, many
of which a few centuries back were, in this respect, almost
savages themselves ; while in the latter case the difficulty is
still greater, for we have to assume the existence of a faculty
which had never been used either by the supposed possessors
of it or by their ancestors.

Let us take, then, the least difficult supposition — that
savages possessed only the mere rudiments of the faculty, such
as their ability to count, sometimes up to ten, but with an
utter inability to perform the very simplest processes of
arithmetic or of geometry — and inquire how this rudimentary
faculty became rapidly developed into that of a Newton, a
La Place, a Gauss, or a Cayley. We will admit that there is
every possible gradation between these extremes, and that
there has been perfect continuity in the development of the
faculty ; but we ask, What motive power caused its develop-
ment ?

It must be remembered we are here dealing solely with
the capability of the Darwinian theory to account for the
origin of the mind, as well as it accounts for the origin of the
body of man, and we must, therefore, recall the essential features
of that theory. These are, the preservation of useful varia-
tions in the struggle for life ; that no creature can be improved
beyond its necessities for the time being ; that the law acts by
life and death, and by the survival of the fittest. We have to
ask, therefore, what relation the successive stages of improve-
ment of the mathematical faculty had to the life or death of
its possessors ; to the struggles of tribe with tribe, or nation
with nation ; or to the ultimate survival of one race and the
extinction of another. If it cannot possibly have had any
such effects, then it cannot have been produced by natural
selection.

It is evident that in the struggles of savage man with the
elements and with wild beasts, or of tribe with tribe, this
faculty can have had no influence. It had nothing to do with

xv DARWINISM APPLIED TO MAN 467

the early migrations of man, or with the conquest and exter-
mination of weaker by more powerful peoples. The Greeks
did not successfully resist the Persian invaders by any aid
from their few mathematicians, but by military training,
patriotism, and self-sacrifice. The barbarous conquerors of
the East, Timurlane and Gengkhis Khan, did not owe their
success to any superiority of intellect or of mathematical
faculty in themselves or their folloAvers. Even if the great
conquests of the Eomans were, in part, due to their systematic
military organisation, and to their skill in making roads and
encampments, which may, perhaps, be imputed to some exercise
of the mathematical faculty, that did not prevent them from
being conquered in turn by barbarians, in whom it was almost
entirely absent. And if we take the most civilised peoples of
the ancient world — the Hindoos, the Arabs, the Greeks, and
the Eomans, all of whom had some amount of mathematical
talent — we find that it is not these, but the descendants of the
barbarians of those days — the Celts, the Teutons, and the
Slavs — who have proved themselves the fittest to survive in
the great struggle of races, although we cannot trace their
steadily growing success during past centuries either to the
possession of any exceptional mathematical faculty or to its
exercise. They have indeed proved themselves, to-clay, to be
possessed of a marvellous endowment of the mathematical
faculty ; but their success at home and abroad, as colonists or
as conquerors, as individuals or as nations, can in no way be
traced to this faculty, since they were almost the last who
devoted themselves to its exercise. We conclude, then, that
the present gigantic development of the mathematical faculty
is wholly unexplained by the theory of natural selection, and
must be due to some altogether distinct cause.

The Origin of the Musical and Artistic Faculties.

These distinctively human faculties follow very closely the
lines of the mathematical faculty in their progressive develop-
ment, and serve to enforce the same argument. Among the
lower savages music, as we understand it, hardly exists, though
they all delight in rude musical sounds, as of drums, tom-toms,
or gongs ; and they also sing in monotonous chants. Almost
exactly as they advance in general intellect, and in the arts

468 DARWINISM

of social life, their appreciation of music appears to rise in
proportion ; and we find among them rude stringed instruments
and whistles, till, in Java, we have regular bands of skilled
performers probably the successors of Hindoo musicians of
the age before the Mahometan conquest. The Egyptians are
believed to have been the earliest musicians, and from them
the Jews and the Greeks, no doubt, derived their knowledge
of the art ; but4t seems to be admitted that neither the latter
nor the Eomans knew anything of harmony or of the essential
features of modern music.1 Till the fifteenth century little
progress appears to have been made in the science or the
practice of music ; but since that era it has advanced with
marvellous rapidity, its progress being curiously parallel with
that of mathematics, inasmuch as great musical geniuses
appeared suddenly among different nations, equal in their
possession of this special faculty to any that have since
arisen.

As with the mathematical, so with the musical faculty,
it is impossible to trace any connection between its possession
and survival in the struggle for existence. It seems to have
arisen as a result of social and intellectual advancement, not
as a cause ; and there is some evidence that it is latent in the
lower races, since under European training native military
bands have been formed in many parts of the world, which
have been able to perform creditably the best modern music.

The artistic faculty has run a somewhat different course,
though analogous to that of the faculties already discussed.
Most savages exhibit some rudiments of it, either in drawing
or carving human or animal figures ; but, almost without
exception, these figures are rude and such as would be
executed by the ordinary inartistic child. In fact, modern
savages are, in this respect hardly equal to those prehistoric
men who represented the mammoth and the reindeer on pieces
of horn or bone. With any advance in the arts of social life,
we have a corresponding advance in artistic skill and taste,
rising very high in the art of Japan and India, but culminating
in the marvellous sculpture of the best period of Grecian
history. In the Middle Ages art was chiefly manifested in
1 See " History of Music," in Eng. Cyc, Science and Arts Division.

xv DARWINISM APPLIED TO MAN 469

ecclesiastical architecture and the illumination of manuscripts,
but from the thirteenth to the fifteenth centuries pictorial art
revived in Italy and attained to a degree of perfection which
has never been surpassed. This revival was followed closely
by the schools of Germany, the Netherlands, Spain, France,
and England, showing that the true artistic faculty belonged
to no one nation, but was fairly distributed among the various
European races.

These several developments of the artistic faculty, whether
manifested in sculpture, painting, or architecture, are evi-
dently outgrowths of the human intellect which have no im-
mediate influence on the survival of individuals or of tribes,
or on the success of nations in their struggles for supremacy
or for existence. The glorious art of Greece did not prevent the
nation from falling under the sway of the less advanced Roman ;
while we ourselves, among whom art was the latest to arise,
have taken the lead in the colonisation of the world, thus
proving our mixed race to be the fittest to survive.

Independent Proof that the Mathematical, Musical, and Artistic
Faculties have not been Developed under the Law of Natural
Selection.

The law of Natural Selection or the survival of the fittest
is, as its name implies, a rigid law, which acts by the life or
death of the individuals submitted to its action. From its
very nature it can act only on useful or hurtful characteristics,
eliminating the latter and keeping up the former to a fairly
general level of efficiency. Hence it necessarily follows that
the characters developed by its means will be present in all
the individuals of a species, and, though varying, will not vary
very widely from a common standard. The amount of varia-
tion we found, in our third chapter, to be about one-fifth or
one-sixth of the mean value — that is, if the mean value were
taken at 100, the variations would reach from 80 to 120, or
somewhat more, if very large numbers were compared. In
accordance with this law we find, that all those characters in
man which were certainly essential to him during his early
stages of development, exist in all savages with some approach
to equality. In the speed of running, in bodily strength, in
skill with weapons, in acuteness of vision, or in power of

470 DARWINISM

following a trail, all are fairly proficient, and the differences
of endowment do not probably exceed the limits of variation
in animals above referred to. So, in animal instinct or intel-
ligence, we find the same general level of development. Every
wren makes a fairly good nest like its fellows ; every fox has
an average amount of the sagacity of its race ; while all the
higher birds and mammals have the necessary affections and
instincts needful for the protection and bringing-up of their
offspring.

But in those specially developed faculties of civilised man
which we have been considering, the case is very different.
They exist only in a small proportion of individuals, while
the difference of capacity between these favoured individuals
and the average of mankind is enormous. Taking first the
mathematical faculty, probably fewer than one in a hundred
really possess it, the great bulk of the population having
no natural ability for the study, or feeling the slightest
interest in it.1 And if we attempt to measure the
amount of variation in the faculty itself between a first-
class mathematician and the ordinany run of people who find
any kind of calculation confusing and altogether devoid of
interest, it is probable that the former could not be estimated
at less than a hundred times the latter, and perhaps a thousand
times would more nearly measure the difference between
them.

The artistic faculty appears to agree pretty closely with
the mathematical in its frequency. The boys and girls who,
going beyond the mere conventional designs of children, draw
what they see, not what they know to be the shape of things ;
who naturally sketch in perspective, because it is thus they
see objects ;. who see, and represent in their sketches, the light
and shade as well as the mere outlines of objects ; and who
can draw recognisable sketches of every one they know, are
certainly very few compared with those who are totally incap-

1 This is the estimate furnished me by two mathematical masters in one of
our great public schools of the proportion of boys who have any special
taste or caj>acity for mathematical studies. Many more, of course, can be
drilled into a fair knowledge of elementary mathematics, but only this small
proportion possess the natural faculty which renders it possible for them ever
to rank high as mathematicians, to take any pleasure in it, or to do any
original mathematical work.

xv DARWINISM APPLIED TO MAN 471

able of anything of the kind. From some inquiries I have
made in schools, and from my own observation, I believe that
those who are endowed with this natural artistic talent do
not exceed, even if they come up to, one per cent of the whole
population.

The variations in the amount of artistic faculty are certainly
very great, even if we do not take the extremes. The grada-
tions of power between the ordinary man or woman "who
does not draw," and whose attempts at representing any
object, animate or inanimate, would be laughable, and the
average good artist who, with a few bold strokes, can produce
a recognisable and even effective sketch of a landscape, a
street, or an animal, are very numerous ; and we can hardly
measure the difference between them at less than fifty or a
hundred fold.

The musical faculty is undoubtedly, in its lower forms,
less uncommon than either of the preceding, but it still differs
essentially from the necessary or useful faculties in that it
is almost entirely wanting in one-half even of civilised men.
For every person who draws, as it were instinctively, there are
probably five or ten who sing or play without having been
taught and from mere innate love and perception of melody
and harmony.1 On the other hand, there are probably about
as many who seem absolutely deficient in musical perception,
who take little pleasure in it, who cannot perceive discords or
remember tunes, and who could not learn to sing or play with
any amount of study. The gradations, too, are here quite
as great as in mathematics or pictorial art, and the special
faculty of the great musical composer must be reckoned many
hundreds or perhaps thousands of times greater than that of
the ordinary " unmusical " person above referred to.

It appears then, that, both on account of the limited
number of persons gifted with the mathematical, the artistic,
or the musical faculty, as well as from the enormous variations
in its development, these mental powers differ widely from .
those which are essential to man, and are, for the most part,
common to him and the lower animals ; and that they could

1 I am ' informed, however, by a music master in a large school that only
about one per cent have real or decided musical talent, corresponding curiously
with the estimate of the mathematicians.

472 DARWINISM

not, therefore, possibly have been developed in him by means
of the law of natural selection.

We have thus shown, by two distinct lines of argument,
that faculties are developed in civilised man which, both in
their mode of origin, their function, and their variations, are al-
together distinct from those other characters and faculties which
are essential to him, and which have been brought to their
actual state of efficiency by the necessities of his existence.
And besides the three which have been specially referred to,
there are others which evidently belong to the same class.
Such is the metaphysical faculty, which enables us to form
abstract conceptions of a kind the most remote from all
practical applications, to discuss the ultimate causes of things,
the nature and qualities of matter, motion, and force, of space
and time, of cause and effect, of will and conscience. Specu-
lations on these abstract and difficult questions are impossible
to savages, who seem to have no mental faculty enabling them
to grasp the essential ideas or conceptions ; yet whenever any
race attains to civilisation, and comprises a body of people who,
whether as priests or philosophers, are relieved from the
necessity of labour or of taking an active part in war or
government, the metaphysical faculty appears to spring sud-
denly into existence, although, like the other faculties we have
referred to, it is always confined to a very limited proportion
of the population.

In the same class we may place the peculiar faculty of wit
and humour, an altogether natural gift whose development
appears to be parallel with that of the other exceptional
faculties. Like them, it is almost unknoAvn among savages,
but appears more or less frequently as civilisation advances and
the interests of life become more numerous and more complex.
Like them, too, it is altogether removed from utility in the
struggle for life, and appears sporadically in a very small per-
centage of the population ; the majority being, as is well
known, totally unable to say a Avitty thing or make a pun
even to save their lives.1

1 In the latter part of his essay on Heredity (pp. 91-93 of the volume of
Essays), Dr. Weismann refers to this question of the origin of "talents" in
man, and, like myself, comes to the conclusion that they could not be developed

DARWINISM APPLIED TO MAN 473

The Interpretation of the Facts.

The facts now set forth prove the existence of a number
of mental faculties which either do not exist at all or exist
in a very rudimentary condition in savages, but appear
almost suddenly and in perfect development in the higher
civilised races. These same faculties are further characterised
by their sporadic character, being well developed only in a
very small proportion of the community ; and by the enormous
amount of variation in their development, the higher mani-
festations of them being many times — perhaps a hundred or
a thousand times — stronger than the lower. Each of these
characteristics is totally inconsistent with any action of the
law of natural selection in the production of the faculties
referred to; and the facts, taken in their entirety, compel
us to recognise some origin for them wholly distinct from that
which has served to account for the animal characteristics —
whether bodily or mental — of man.

under the law of natural selection. He says : " It may be objected that, in
man, in addition to the instincts inherent in every individual, special indi-
vidual predispositions are also found, of such a nature that it is impossible
they can have arisen by individual variations of the germ-plasm. On the
other hand, these predispositions — which we call talents — cannot have arisen
through natural selection, because life is in no way dependent on their presence,
and there seems to be no way of explaining their origin except by an assump-
tion of the summation of the skill attained by exercise in the course of each
single life. In this case, therefore, we seem at first sight to be compelled to
accept the transmission of acquired characters." Weismann then goes on to
show that the facts do not support this view ; that the mathematical, musical,
or artistic faculties often appear suddenly in a family whose other members
and ancestors were in no way distinguished ; and that even when hereditary
in families, the talent often appears at its maximum at the commencement or
in the middle of the series, not increasing to the end, as it should do if it
depended in any way on the transmission of acquired skill. Gauss was not the
son of a mathematician, nor Handel of a musician, nor Titian of a painter, and
there is no proof of any special talent in the ancestors of these men of genius,
who at once developed the most marvellous pre-eminence in their respective
talents. And after showing that such great men only appear at certain stages
of human development, and that two or more of the special talents are not
unfrequently combined in one individual, he concludes thus — ■

" Upon this subject I only wish to add that, in my opinion, talents do not
appear to depend upon the improvement of any special mental quality by
continued practice, but they are the expression, and to a certain extent the
bye -product,, of the human mind, which is so highly developed in all
directions. "

It will, I think, be' admitted that this view hardly accounts for the
existence of the highly peculiar human faculties in question.

474 DARWINISM

The special faculties we have been discussing clearly point
to the existence in man of something which he has not derived
from his animal progenitors — something which we may best
refer to as being of a spiritual essence or nature, capable of
progressive development under favourable conditions. On
the hypothesis of this spiritual nature, superadded to the
animal nature of man, we are able to understand much that
is otherwise mysterious or unintelligible in regard to him,
especially the enormous influence of ideas, principles, and
beliefs over his whole life and actions. Thus alone we can
understand the constancy of the martyr, the unselfishness of
the philanthropist, the devotion of the patriot, the enthusiasm
of the artist, and the resolute and persevering search of
the scientific worker after nature's secrets. Thus we may
perceive that the love of truth, the delight in beauty, the
passion for justice, and the thrill of exultation with which we
hear of any act of courageous self-sacrifice, are the workings
within us of a higher nature which has not been developed
by means of the struggle for material existence.

It will, no doubt, be urged that the admitted continuity of
man's progress from the brute does not admit of the introduc-
tion of new causes, and that we have no evidence of the
sudden change of nature which such introduction would bring
about. The fallacy as to new causes involving any breach of
continuity, or any sudden or abrupt change, in the effects, has
already been shown ; but we will further point out that there
are at least three stages in the development of the organic
world when some new cause or power must necessarily have
come into action.

The first stage is the change from inorganic to organic,
when the earliest vegetable cell, or the living protoplasm out
of which it arose, first appeared. This is often imputed to
a mere increase of complexity of chemical compounds ; but
increase of complexity, with consequent instability, even if we
admit that it may have produced protoplasm as a chemical
compound, could certainly not have produced living protoplasm
— protoplasm which has the power of growth and of reproduc-
tion, and of that continuous process of development which has
resulted in the marvellous variety and complex organisation of
the whole vegetable kino;dom. There is in all this something;

xv DARWINISM APPLIED TO MAN 475

quite beyond and apart from chemical changes, hoAvever
complex ; and it has been well said that the first vegetable
cell was a new thing in the world, possessing altogether new
powers — that of extracting and fixing carbon from the carbon-
dioxide of the atmosphere, that of indefinite reproduction,
and, still more marvellous, the power of variation and of
reproducing those variations till endless complications of
structure and varieties of form have been the result. Here,
then, we have indications of a new power at work, which we
may term vitality, since it gives to certain forms of matter
all those characters and properties which constitute Life.

The next stage is still more marvellous, still more completely
beyond all possibility of explanation by matter, its laws and
forces. It is the introduction of sensation or consciousness,
constituting the fundamental distinction between the animal
and vegetable kingdoms. Here all idea of mere complication
of structure producing the result is out of the question. We
feel it to be altogether preposterous to assume that at a certain
stage of~ complexity of atomic constitution, and as a necessary
result of that complexity alone, an ego should start into
existence, a thing that feels, that is conscious of its own existence.
Here we have the certainty that something new has arisen, a
being whose nascent consciousness has gone on increasing in
power and definiteness till it has culminated in the higher
animals. No verbal explanation or attempt at explanation —
such as the statement that life is the result of the molecular
forces of the protoplasm, or that the whole existing organic
universe from the amseba up to man was latent in the fire-mist
from which the solar system was developed — can afford any
mental satisfaction, or help us in any way to a solution of the
mystery.

The third stage is, as we have seen, the existence in man
of a number of his most characteristic and noblest faculties,
those which raise him furthest above the brutes and open up
possibilities of almost indefinite advancement. These faculties
could not possibly have been developed by means of the same
laws which have determined the progressive development of the
organic world in general, and also of man's physical organism.1

1 For an earlier discussion of this subject, with some wider applications, see
the author's Contributions to the Theory of Natural Selection, chap. x.

476 DARWINISM chap.

These three distinct stages of progress from the inorganic
world of matter and motion up to man, point clearly to an
unseen universe — to a world of spirit, to which the world of
matter is altogether subordinate. To this spiritual world we
may refer the marvellously complex forces which we know
as gravitation, cohesion, chemical force, radiant force, and
electricity, without which the material universe could not
exist for a moment in its present form, and perhaps not at all,
since without these forces, and perhaps others which may be
termed atomic, it is doubtful whether matter itself could have
any existence. And still more surely can we refer to it those
progressive manifestations of Life in the vegetable, the
animal, and man — which we may classify as unconscious,
conscious, and intellectual life, — and which probably depend
upon different degrees of spiritual influx. I have already
shown that this involves no necessary infraction of the law of
continuity in physical or mental evolution ; whence it follows
that any difficulty we may find in discriminating the inorganic
from the organic, the lower vegetable from the lower animal
organisms, or the higher animals from the lowest types of
man, has no bearing at all upon the question. This is to
be decided by showing that a change in essential nature (due,
probably, to causes of a higher order than those of the
material universe) took place at the several stages of progress
which I have indicated ; a change which may be none the less
real because absolutely imperceptible at its point of origin,
as is the change that takes place in the curve in which a body
is moving when the application of some new force causes the
curve to be slightly altered.

Concluding Remarks.

Those who admit my interpretation of the evidence now ad-
duced— strictly scientific evidence in its appeal to facts which
are clearly what ought not to be on the materialistic theory —
will be able to accept the spiritual nature of man, as not in
any way inconsistent with the theory of evolution, but as de-
pendent on those fundamental laws and causes which furnish
the very materials for evolution to work with. They will
also be relieved from the crushing mental burthen imposed
upon those who — maintaining that we, in common with the

xv DARWINISM APPLIED TO MAN 477

rest of nature, are but products of the blind eternal forces of
the universe, and believing also that the time must come when
the sun will lose his heat and all life on the earth necessarily
cease — have to contemplate a not very distant future in which
all this glorious earth — which for untold millions of years has
been slowly developing forms of life and beauty to culminate
at last in man — shall be as if it had never existed ; who are
compelled to suppose that all the slow growths of our race
struggling towards a higher life, all the agony of martyrs, all
the groans of victims, all the evil and misery and undeserved
suffering of the ages, all the struggles for freedom, all the
efforts towards justice, all the aspirations for virtue and the
wellbeing of humanity, shall absolutely vanish, and, " like the
baseless fabric of a vision, leave not a wrack behind."

As contrasted with this hopeless and soul-deadening belief,
we, who accept the existence of a spiritual world, can look
upon the universe as a grand consistent whole adapted in all
its parts to the development of spiritual beings capable of
indefinite life and perfectibility. To us, the whole purpose,
the only raison d'etre of the world — with all its complexities
of physical structure, with its grand geological progress, the
slow evolution of the vegetable and animal kingdoms, and
the ultimate appearance of man — was the development of the
human spirit in association with the human body. From the
fact that the spirit of man — the man himself — is so developed,
we may well believe that this is the only, or at least the best,
way for its development ; and we may even see in what is
usually termed " evil " on the earth, one of the most efficient
means of its growth. For we know that the noblest
faculties of man are strengthened and perfected by struggle
and effort ; it is by unceasing warfare against physical evils
and in the midst of difficulty and danger that energy,
courage, self-reliance, and industry have become the common
qualities of the northern races ; it is by the battle with
moral evil in all its hydra-headed forms, that the still
nobler qualities of justice and mercy and humanity and self-
sacrifice have been steadily increasing in the world. Beings
thus trained and strengthened by their surroundings, and
possessing latent faculties capable of such noble development,
are surely destined for a higher and more permanent exist-

478 DARWINISM CHAP, Xv

ence ; and we may confidently believe with our greatest living
poet —

That life is not as idle ore,

But iron dug from central gloom,

And heated hot with burning fears,
And dipt in baths of hissing tears,

And batter'd with the shocks of doom

To shape and use.

We thus find that the Darwinian theory, even when
carried out to its extreme logical conclusion, not only does not
oppose, but lends a decided support to, a belief in the spiritual
nature of man. It shows us how man's body may have been
developed from that of a lower animal form under the law of
natural selection ; but it also teaches us that we possess
intellectual and moral faculties which could not have been so
developed, but must have had another origin ; and for this
origin we can only find an adequate cause in the unseen
universe of Spirit.

INDEX

Abbott, Dr. C. C, instability of

habits of birds, 76
on American water - thrushes

(Seiurus), 117
Mr., drawings of caterpillars

and their food plants, 203
Accessory plumes, development and

display of, 293
Acclimatisation, 94
AchatinelKcUe, Gulick on variations

in, 147
Acquired characters, non-heredity of,

440
Acrseidas, mimicry of, 247
Adaptation to conditions at various

periods of life, 112
Adolias dirtea, sexual diversity of,

271
iEgeriidae, mimicry by, 240
Agaristidte, mimicry of, 246
Agassiz, on species, 5

on North American weeds, 15.
Agelaeus phceniceus, diagram showing

variations of, 56 ; propor-
tionate numbers which vary,

64
Albatross, courtship of great, 287
Allen, Mr. Grant, on forms of leaves,

133
on degradation of wind-fertilised

from insect-fertilised flowers,

325 (note)
on insects and flowers, 332
on production of colour through

the agency of the colour

sense, 334
Mr. J. A., on the variability

of birds, 50

Allen, Mr. J. A., on colour as in-
fluenced by climate, 228
Alluring coloration, 210
American school of evolutionists, 420
Anemone nemorosa, variability of, 78
Animal coloration, a theory of, 288
general laws of, 296
intelligence, supposed action of,

425
characteristics of man, 454
Animals, the struggle among, 18

wild, their enjoyment of life,

39
usually die painless deaths, 38
constitutional variation of, 94
uses of colours of, 134
supposed effects of disuse in

wild, 415
most allied to man, 450
Antelopes, recognition marks of, 219
Anthrocera filipendula inedible, 235
Apples, variations of, 87
Arctic animals, supposed causes of

white colour of, 191
Argyll, Duke of, on goose reared by

a golden eagle, 75
Artemia salina and A. milhausenii,

426
Asclepias curassavica, spread of, 28
Asses running wild in Quito, 28
Attractive fruits, 306
Australia, spread of the Cape-weed
in, 29
fossil and recent mammals of,
392
Azara, on cause of horses and cattle
not running wild in Paraguay,
19
Azores, flora of, supports aerial trans-
mission of seeds, 368

480

INDEX

Baker, Mr. J. G., on rarity of spiny

plants inJVEauritius, 432
Ball, Mr., on cause of late appearance

of exogens, 400
Barber, Mrs. , on variable colouring of
pupae of Papilio nireus, 197
on protective colours of African
sun-birds, 200
Barbs, 91
Barriers, importance of, in questions

of distribution, 341
Bates, Mr. H. W., on varieties of
butterflies, 44
on inedibility of Heliconidse,

234
on a conspicuous caterpillar,

236
on mimicry, 240, 243, 249
Batlimism or growth-force, Cope on,

421
Beddard, Mr. F. E., variations of
earth-worms, 67
on plumes of bird of paradise,
292
Beech trees, aggressive in Denmark,

21
Beetle and wasp (figs.), 259
Beetle, fossil in coal measures of

Silesia, 404
Beginnings of important organs, 128
Belt, Mr., on leaf-like locust, 203
on birds avoiding Heliconidas,
234
Belt's frog, 266
Birds, rate of increase of, 25
how destroyed, 26
variation among, 49
variation of markings of, 52
variation of wings and tails of,

53
diagram showing variation of

tarsus and toes, 60
iise of structural peculiarities

of, 135
eggs, coloration of, 212
recognition marks of, 222
and butterflies, white in tropical

islands, 230
sometimes seize inedible butter-
■ flies, 255
mimicry among, 263

Birds, sexual coloration of, 275

cause of dull colour of female,

277
choice of female not known to
be determined by colour, etc.,
285
decorative plumage of, 285
antics of unornamented, 287
which fertilise flowers, 319
colours of, not dependent on the

colours of flowers, 336
no proof of aesthetic tastes in,

336
dispersal of, 355
and insects at sea, 357
of oceanic islands, 358
carrying seeds on their feet,

361
ancestral forms of, 407
Birthplace, probable, of man, 459
Bombyx regia, protective form of

larva of, 210
Boyd Dawkins, on development of
deer's, horns, 389
on origin of man, 456
Brady, Mr. George, on protective

colouring of star-fishes, 209
Brain development, progressive, 390
Brains of man and apes, 452
Branner, Mr. J. C, on supposed
proofs of glaciation in Brazil,
370
Brazil, supposed proof of glaciation

in, 370
Brewer, Professor W. H., on want of
symmetry in colours of
animals, 217
Bromelia, animals inhabiting leaves

of, 118
Bronn, Professor, on supposed useless-
ness of variations of ears and
tails, 136
Butler, Mr. A. G., on inedibility of
conspicuous caterpillars, 237
Butterflies, varieties of, 44

small, of Isle of Man, 106
special protective colouring of,

206
recognition by, 226
inedibility of some, 234
mimicry among, 240, 249
colour development of, 274
sexual coloration of, 271

INDEX

481

C

Caddis-fly larvte inhabiting bro-

melia leaves, 118
Callopliis, harmless mimicking poison-
ous species, 262
Candolle, Alp. de, on variation in

oaks, 77
on variability of Papaver

bracteatnm, 79
Cardinalis virginianus, diagram show-
ing proportionate numbers

which vary, 65 ; variations

of, 58
Carpenter, Dr. W. B., on variation in

the Foraminifera, 43
Carriers, 91
Caterpillars, resemblance of, to their

food plants, 203-205
inedible, 236
Cattle, how they prevent the growth

of trees, 18
increase of, in St. Domingo,

Mexico, and the pampas, 27
Ceylon, spread of Lantana mixta in, 29
Chaffinch, change of habit of, in New

Zealand, 76
Chambers, Robert, on origin of species,

3
Chance rarely determines survival,

123
Change of conditions, utility of, 326
Characters, non-adaptive, 131

transferred from useless to use-
ful class, 1 32
Charaxes psaphon persecuted by a

bird, 235
Chile, numerous red tubular flowers

in, 320
Chimpanzee, figure of, 454
Clark, Mr. Edwin, on cause of absence

of forests on the pampas, 23
on the struggle for life in the

South American valleys, 24
Cleistogamous flowers, 322
Close interbreeding, supposed evil

results of, 326
Clover, white, spread of, in New

Zealand, 28
Co-adaptation of parts by variation,

no real difficulty, 418
Cobra, use of hood of, 262

Coccinella mimicked by grasshopper,

(figure), 260
Collingwood, Mr., on butterflies re-
cognising their kind, 226
Coloration, alluring, 210

of birds' eggs, 212

a theory of animal, 288
Colour correlated with sterility, 169

correlated with constitutional
peculiarities, 170

iii nature, the problem to be
solved, 188

constancy, in animals indicates
utility, 189

and environment, 190

general theories of animal, 193

animal, supposed causes of, 193

obscure, of many tropical ani-
mals, 194

produced by surrounding ob-
jects, 195

adaptations, local, 199

for recognition, 217

of wild animals not quite sym-
metrical, 217 (note)

as influenced by locality or
climate, 228

development in butterflies, 274

more variable than habits, 278

and nerve distribution, 290

aud tegumentary appendages,
291

of -flowers, 308

change of, in flowers when fertil-
ised, 317

in nature, concluding remarks
on, 299, 333

of fruits, 304

of flowers growing together con-
trasted, 318
Complexity of flowers due to alternate
adaptation to insect and self-
fertilisation, 328
Composite, a, widely dispersed with-
out pappus, 367
Confinement, affecting fertility, 154
Continental and oceanic areas, 346
Continents and oceans cannot have
changed places, 345

possible connections between,
349
Continuity does not prove identity of
origin, 463

2 I

482

INDEX

Cope, Dr. E. D., on non - adaptive
characters, 131
on fundamental laws of growth,

420
on bathmism or growth-force,

421
on use producing structural

change, 422
on law of centrifugal growth,

422
on origin of the feet of ungu-
lates, 423
on action of animal intelligence,
425
Correlations in pigeons, horses, etc.,

140
Corvus frugilegus, 2

corone, 2
Coursers, figures of secondary quills,

224
Cowslip, two forms of, 157
Crab, sexual diversity of colour of,

269
Cretaceous period, dicotyledons of,

400
Crisp, Dr., on variations of gall
bladder and alimentary canal,
69
Crosses, a cause of variation, 99

reciprocal, 155
Cross-fertilisation, modes of securing,
310
difference in, 155
Crossing and changed conditions,

parallelism of, 166
Cruciferse, variations of structure in,

80
Cuckoo, eggs of, 216
Cuckoos mimiek hawks, 263
Cultivated plants, origin of useful,

97
Curculionidse mimicked by various

insects (figs.), 260
Curves of variation, 64

Dana, Professor, on the permanence

of continents, 342
Danaidne little attacked by mites, 235
mimicry of, 246

Darwin, change of opinion effected

by, 8
the Newton of Natural History,

9
his view of his own work, 10
on the enemies of plants, 16
on fir-trees destroyed by cattle,

17
on change of plants and animals

caused by planting, 18
on absence of wild cattle in

Paraguay, 19
on cats and red clover, 20
on variety of plants in old turf,

35
on the beneficent action of the

struggle for existence, 40
on variability of wild geraniums,

79
on variability of common species,

80
his non-recognition of extreme

variability of wild species, 82
on races of domestic pigeon,

90
on constitutional variation in

plants, 95
on unconscious selection, 96
on a case of divergence, 105
on advantage of diversification

of structure in inhabitants of

one region, 110
on species of plants in turf, 110
on isolation, 119
on origin of mammary glands,

129
on eyes of flatfish, 129
on origin of the eye, 130
on useless characters, 131
on use of ears and tails, 136
on disappearance of sjjorts, 140
on tendency to vary in one

direction, 141
on rare perpetuation of sports,

142
on utility of specific characters,

142 (note)
on importance of biological en-
vironment, 148
on variable fertility of plants,

155
on fertile hybrids among plants,
164

INDEX

483

Darwin, on correlation of sterility and
colour, 169
on selective association, 172
on infertility and natural selec-
tion, 174
on cause of infertility of hybrids,

185
on white tail of rabbit, 218
on conspicuous caterpillars, 236
on sexual selection in insects,

274
on decorative plumage of male

birds, 285
on development of ocelli, 290
on value of cross-fertilisation, 309
on limits to utility of inter-
crossing, 326
on flowers due to insects, 332
on oceanic islands, 342
on effects of disuse in domestic

animals, 415, 435
on direct action of environment,

419
on unintelligibility of theory of
retardation and acceleration,
421 (note)
on origin of man's moral nature,

461
Mr. George, on intermarriages
of British aristocracy, 326
Darwinian theory, statement of, 10
not opposed to spiritual nature
of man, 478
Dawkins, Professor Boyd, on develop-
ment of deer's horns, 389
on recent origin of man, 456
Dawson, Sir W., on determination of
fossil plants by leaves, 398
(note)
Death of wild animals usually pain-
less, 38
De Candolle, definition of species, 1
on difficulty of naturalising

plants, 15
on war between plants, 16
on origin of useful cultivated
plants, 97
Deer's horns, development of, 389
Degeneration, 121
Delbouuf's law of variation, 141
Dendrseca coronata, variation of wing-
feathers of, 51
Denmark, struggle between trees in, 20

Denudation, evidences of, 379

Desert animals, colour of, 192

Deserts, effect of goats and camels in
destroying vegetation in, 17

Development and display of acces-
sory plumes, 293

Diadema anomala, 271

misippus, great diversity of sexes
in, 271

Diaphora mendica mimics Spilosoma
menthrasti, 249

Difficulties in the facts of fertilisation
of flowers, 325

Dimorphism and trimorphism, 156

Dippers, probable origin of, 116

Disease and markings, 290

Diseases common to man and animals,
449

Display of decorative plumage, 287

Distribution of organisms should be
explained by theory of de-
scent, 338
conditions which have deter-
mined the, 341
of marsupials, 350
of tapirs, 352

Disuse, effects of, among wild animals,
415
no proof that the effects of, are
inherited, 417

Divergence of character, 105-109

leads to maximum of forms of
life in each area, 109

Diversity of fauna and flora with geo-
graphical proximity, 339

Dixon, Mr. C, changed habits of
chaffinch in New Zealand, 76

Dogs, origin of, 88
varieties of, 89

Dolichonyx oryzivorus, diagram show-
ing variations of, 55

Domestic animals, varieties of, 88

Draba verna, varieties of, 77

Dress of men not determined by
female choice, 286

Dust from Krakatoa, size of particles
• of, 363

E

Eastern butterflies, variation of, 45
Eaton, Rev. A. E., on Kerguelen in-
sects, 106

484

INDEX

Edwards, Mr. W. H., on dark forms

of Papilio turmis, 248
Eggs protectively coloured, 214, 215
theory of varied colours of,

216
Elaps mimicked by harmless snakes,

261
Embryonic development of man and

other mammalia, 448
Ennis, Mr. John, on willows driving

out watercresses from rivers

of New Zealand, 24
Entomostraca, in bromelia leaves, 118
Environment never identical for two

species, 149
direct action of, 418
direct influence of, 426
as initiator of variations, 436
action of, overpowered by natu-
ral selection, 437
Ethical aspect of the struggle for

existence, 36
Euchelia jacobete inedible, 235
Everett, Mr. A., on a caterpillar re-
sembling moss, 205
Evidence of evolution that may be

expected among fossil forms,

380
Evolutionists, American school of,

420
Exogens, possible cause of sudden late

appearance of, 400
External differences of man and apes,

453
Extinct animals, number of species of,

376
Extinction of large animals, cause of,

394
Eye, origin of, 130
Eyes, explanation of loss of in cave

animals, 416

F

Facts of natural selection, summary

of, 122
Falcons illustrating divergence, 108
and butcher birds, hooked and

toothed beaks of, 422
Fantails, 91
Female birds, why often dull coloured,

277

Female birds, what their choice of
mates is determined by, 286
butterflies, why dull coloured,

272
brighter than male bird, 281
choice a doubtful agent in selec-
tion, 283
preference neutralised by natural
selection, 294
Fertility of domestic animals, 154
Flatfish, eyes of, 129
Flesh-fly, enormous increase of, 25
Floral structure, great differences of, in
allied genera and species, 329
Flowers, variations of, 88
colours of, 308
with sham nectaries, 317
changing colour when fertilised,

317
adapted to bees or to butterflies,

318
contrasted colours of, at same

season and locality, 318
fertilisation of, by birds, 319
self-fertilisation of, 321
once insect-fertilised now self-
fertile, 323
how the struggle for existence

acts among, 328
repeatedly modified during

whole Tertiary period, 331
the product of insect agency, 332
Forbes, Mr. H. O., on protective
colour of a pigeon, 200
on spider imitating birds' drop-
ping, 211
Fossil shells, complete series of transi-
tional forms of, 381
crocodiles afford evidence of

evolution, 383
horses in America, 3S6
and living animals, local rela-
tions of, 391
Fowl, early domestication of, 97
Frill-back, Indian, 93
Frog inhabiting bromelia leaves, 118
Fruits, use of characters of, 133
colours of, 304
edible or attractive, 306
poisonous, 307
Fulica atra, protectively coloured eggs

of, 215
Fulmar petrel, abundance of, 30

INDEX

485

G-

Gallinace.'E, ornamental plumes of,

292
Galton, Mr. F. , diagrams of variability
used by, 74
on markings of zebra, 220 (note)
on regression towards medio-
crity, 414
theory of heredity by, 443 (note)
on imperfect counting of the
Damaras, 464
Gaudry on extinct animals at Pikermi,

377
Gay, Mons. T., on variations of struc-
ture in Cruciferas, 80
Gazella scemmerringi (figure), 219
Gazelles, recognition marks of, 218
Geddes, Professor, on variation in
plants, 428
objection to theory of, 430
Geikie, Dr. Archibald, on formation
of marine stratified rocks, 344
Geoffroy-St. Hilaire, on species, 6
Geological evidences of evolution,
376, 381
record, causes of imperfection

of, 379
distribution of insects, 403
antiquity of man, 455
Ghost-moth, colours of, 270
Glaciation, no proofs of, in Brazil, 370
Glow-worm, light a warning of in-
edibility, 287
Gomphia oleasfolia, variability of, 79
Goose eating flesh, 75
Gosse, Mr. P. H., on variation in the
sea-anemones, 43
on sea-anemone and bullhead,
265
Gould, Mr., on colours of coast and

inland birds, 228
Grant Allen, on forms of leaves, 133

on insects and flowers, 332
Graphite in Laurentian implies abun-
dant plant life, 398
Gray, Dr. Asa, on naturalised plants
in the United States, 110
Dr. J. E., on variation of skulls
of mammalia, 71
Great fertility not essential to rapid
increase, 30

Great powers of increase of animals,

27
Green colour of birds in tropical

forests, 192
Grouse, red, recent divergence of, 106
Gulick, Rev. J. T., on variation of
land-shells, 43
on isolation and variation, 147,

150
on divergent evolution, 148

H

Habits of animals, variability of,

74
Hairy caterpillars inedible, 237
Hanbury, Mr. Thomas, on a remark-
able case of wind conveyance

of seed, 373 (note)
Hansten-Blangsted, on succession of

trees in Denmark, 21
Harvest mice, prehensile tails of

young, 136
Hawk weed, species and varieties of

British, 77
Hector, Sir James, use of horns of

deer, 137
Heliconida?, warning colours of, 234

mimicry of, 240
Helix nemoralis, varieties of, 43
hortensis, varieties of, 43
Hemsley, Mr., on rarity of spines in

oceanic islands, 432
Henslow, Professor G., on vigour of

self-fertilised plants, 323
on wind-fertilised as degrada-
tions from insect -fertilised

flowers, 324
on origin of forms and structures

of flowers, 434 (note)
Herbert, Dean, on species, 6
on plant hybrids, 164
Herbivora, recognition marks of, 218
Heredity, 11

Weismann's theory of, 437
Herschel, Sir John, on species, 3
Hooker, Sir Joseph, on attempts at

naturalising Australian plants

in New Zealand, 16
Home, Mr. C, on inedibility of an

Indian locust, 267
Horns of deer, uses of, 136

486

INDEX

Horse tribe, pedigree of, 384
ancestral forms of, 386
Humming-birds, recognition marks of,

226
Hnth, Mr., on close interbreeding,

160
Huxley, Professor on the struggle
for existence, 37
on fossil crocodiles, 383
on anatomical peculiarities of

tlie horse tribe, 384
on development of vertebrates,

448
on early man, 456
on brains of man and the gorilla,
457
Hybridity, remarks on facts of, 166

summary on, 184
Hybrids, infertility of, supposed test
of distinct species, 152
fertility of, 159
fertile among animals, 162
between sheep and goat, 162
fertile between distinct species

of moths, 163
fertile among plants, 163
Hymenopus bicornis, resembling
flower, 212

Icterus Baltimore, diagram showing
proportionate numbers which
vary, 63
Imitative resemblances, how pro-
duced, 205
Increase of organisms in a geometrical

ratio, 25
Inedible fruits rarely coloured, 308
Insect and self-fertilisation, alterna-
tion of, in flowers, 328
Insect-fertilisation, facts relating to,

316
Insects, coloured for recognition, 226
warning colours of, 233
sexual coloration of, 269
importance of dull colours to

female, 272
visiting one kind of flower at

a time, 318
and flowers, the most brilliant
not found together, 335

Insects, no proof of love of colour by,
336
and birds at sea, 357
in mid-ocean, 359
at great altitudes, 360
geological distribution of, 403
ancestral in Silurian, 405
fossil support evolution, 405
Instability of useless characters, 138
Instinct, the theory of, 441
Insxilar organisms illustrate powers

of dispersal, 354
Interbreeding, close, injurious effects
of, 160
supposed evil results of close,
326
Intercrossing, swamping effects of,
142
not necessarily useful, 325
Intermediate forms, why not found,

380
Islands, all oceanic are volcanic or

coralline, 342
Isle of Man, small butterflies of, 106
Isolation, the importance of, 119
to prevent intercrossing, 144
by variations of habits, etc., 145
Rev. J. G. Gulick on, 147
when ineffective, 150
Ituna Ilione and Thyridia megisto,
figures of wings of, 251

Jacobin, 93

Jenyns, Rev. L., on internal varia-
tions of mammalia, 69

Jordan, Mons. A., on varieties of
Draba verna, 77

Judd, Professor, on dust fallen at
Genoa, 363
on Hungarian fossil lacustrine
shells, 381

K

Kerguelen Island, wingless insects

of, 106
Kerivoula picta, protective colour of,

201

INDEX

487

Kerner, Professor, on use of external
characters of plants, 133
on seeds found on glaciers, 366

Kingfishers illustrating divergence of
character, 109

Lacebta muralis, diagram of varia-
tion of, 47
Lagopus scoticus, origin of, 107
Lamarck, on origin of species, 3
Land debris deposited near coasts,

343
and ocean, diagram showing

comparative height and depth

of, 345
Large animals, cause of extinction of,

394
Larva? of moths, variability of, 46
Laughers, Frill-backs, Nuns, Spots,

and Swallows, 93
Law of relation of colour and nest,

278, 279
Laws of animal coloration, 296
Lawson Tait, on uses of tails, 136
Leaf-butterflies, 207
Leguminosa?, rare in oceanic islands,

368
Lemuria, an unsound hypothesis, 354
Lepidoptera, variation of, 44
Leyden Museum, diagram showing

variability of birds in, 61
Life, Weismann on duration of, 437

(note)
Limenitis misippus mimics Danais

archippus, 248
Ursula mimics Papilio philenor,

248
Linnseus, on rapid increase of the

flesh-fly, 25
Livingstone, his sensations when

seized by a lion, 38
Lizards, variation among, 46

diagram of variation of, 48
sexual colours of, 281
Local colour adaptations, 199
Locusts with warning colours inedible,

267
Longicorus mimic Malacoderms, 257
Low, Mr., on effects of close inter-
breeding, 160

Low, Mr., on fertile crosses between
sheep and goat, 162
on selective association, 172
Low forms of life, continued exist-
ence of, explained, 114
forms, persistence of, 121
temperature of tropics not
needed to explain plant dis-
persal, 370
Lower types, extinction of, among the

higher animals, 114
Lubbock, Sir John, on forms of
leaves, 133
on imperfect counting of early
man, 464
Lyell, Sir Charles, on variation of
species, 4
on the shifting of continents,
342

M

Madagascar and New Zealand,

347
Madeira, wingless beetles of, 105
Maize, origin of, 98
Male rivalry, a real cause of selec-
tion, 283
Males of many animals fights to-
gether, 282
Malm, .on eyes of flatfish, 129
Mammalia, variation of, 65

sexual colours of, 281, 282
afford crucial tests of theories of

distribution, 353
early forms of, 407
geological distribution of, 408
Mammary glands, supposed origin

of, 129
Man, summary of animal character-
istics of, 454
geological antiquity of, 455
early remains of, in California,

456
probably as old as the Miocene,

457
probable birthplace of, 459
origin of moral and intellectual

nature of, 461
possesses mental qualities not
derived exclusively from his
animal progenitors, 474

INDEX

Man's body that of an animal, 444
development similar to that of

animals, 449
structure compared with that of
the anthropoid apes, 451
Mania typica refused by lizards, 238
Mantidae resembling flowers, 212
Marcgravia nepenthoides fertilised by

birds (woodcut), 320
Marine animals, protective resem-
blance among, 208
with warning colours, 266
Marsh, Mr., on destructiveness to
vegetation of goats and camels,
17
Professor 0., on the develop-
ment of the horse tribe, 386
on brain development of Tertiary

mammals, 391
on specialised forms dying out,
395
Marsupials, distribution of, 350
Mathematical faculty, the origin of
the, 464
how developed, 466
not developed by law of natural
selection, 469
Mathematics, late development of,

465
Meldola, Professor E., on variable pro-
tective colouring, 196
on mimicry among British

moths, 249 (note)
on an extension of the theory
of mimicry, 255 (note)
Melons, variations of, 87
Methona psidii and Leptalis orise

(figs.), 241
Meyer, Dr. A. B., on mimicry of

snakes, 262
Milne Edwards, on variation of

lizards, 46
Mimicking birds deceive naturalists,
264
butterfly, figure of, 241
Mimicry, 239

how it has been produced, 242

among protected genera, 249

extension of, 255

in various orders of insects, 257

among vertebrata, 261

among birds, 263

objections to theory of, 264

Mineral particles carried by wind,

363
Miocene fossils of North America,

378
Missing links, character of, 380
Mivart, Dr. St. George, on variation
of ribs and dorsal vertebra,
69
on supposed useless characters,

138 (note)
on resemblance of man and apes,
451
Modifications for special purposes,

113
Mongrels, sterility of, 165
Monkeys affected by medicines as are

men, 450
Monocotyledons degradations from
dicotyledons, 325 (note)
scarcity of, in Rocky Mountains,

401
scarcity of, in Alpine flora, 401
Moral nature of man, origin of,

461
Morse, Professor E. T., on protective
colouring of marine mollusca,
209
Moseley, Professor, on jirotective
resemblance among marine
animals, 208
on courtship of Great Albatross,
287
Moths, protected groups of, 235
Mountains, remote, with identical

plants, 369
Miiller, Dr. Fritz, on inhabitants of
bromelia leaves, 118
on butterfly, deceived by its

mimic, 245
his explanation of mimicry
among protected genera, 252
Dr. Hermann, on variability of
Myosurus minimus, 78
Murray, Mr. John, on bulk of land
and ocean, 344
on quartz particles on ocean

floor, 363
Rev. R. P., variation in the
neuration of butterflies' wings,
45
Musical and artistic faculties, origin

of, 467
Myosurus minimus, variability of, 78

INDEX

489

N

Natural selection with changed
and unchanged conditions,
103
and sterility, 173
overpowers effects of use and

disuse, 435
the most important agency in
modifying species, 444
Naturalist deceived by a mimicking
insect, 259
by mimicking birds, 264
Naudin, M., on varieties of melons,

87
Nectarinea amethystina, protective

colouring of, 201
Nestor notabilis, variation of habits

of, 75
Nests of birds influence the colour of

females, 278
New species, conditions favourable to

origin of, 115
Newton, Professor A., on fertile hybrid

ducks, 162
New Zealand, European plants in, 15
spread of white clover in, 28
effects of introduced plants in,

29
native rat and fly exterminated

by European species, 34
many plants of, incapable of

self-fertilisation, 321
fauna of, 348
few spiny plants in, 433
Nocturnal animals, colours of, 193
Non-adaptive characters, instability

of, 138
Normandy pigs, fleshy appendages to

jaws of, 139
North America, Miocene fossils of, 378
Northern plants in southern hemi-
sphere, 368
Nostus Borbonicus, variability of, 80
Number of individuals which vary, 62
Nutmeg, how dispersed, 307
Nuts, not meant to be eaten, 305

O

Oaks, great variability of, 78
Objections to Darwin's theory, 126

Ocean floor, deposits on, 343
Oceanic animals, colours of, 193
and continental areas, 346
islands have no mammals or
batrachia, 342
Oceans, the permanence of, 341
CEdicnemus, figures of wings of, 223
Opthalrnis lincea and Artaxa simulans

(figs.), 247
Orang-utans, variations of skull of,

69
Orchidese, why scarce on oceanic

islands, 367
Orchis jjyramidalis, mode of fertilisa-
tion of, 314
figures illustrating fertilisation
of, 315
Organic development, three stages of,
involving new cause or power,
474
world, the development of, im-
plies a spiritual world, 476
Organisation, advance of, by natural
selection, 120
degradation in, 121
Origin of species, objections, 7
of accessory plumes, 291
Orioles mimicking honeysuckers, 263
Ornamental plumes and vitality, 293

Pachtrhtnchi subjects of mimicry,

261
Pampas, effects of drought in, 23
Papaver bracteatum, variability of,

79
Papilio, use of forked tentacle of larva?

of, 210
protected groups of, 235
mimicry of, 247
Paraguay, absence of wild cattle and

horses, 19
Parnassia jmlustris, sham nectaries

of, 317
Parrot, change of habits of New

Zealand, 75
Parus, species of, illustrate divergence,

of character, 107
Passenger •• pigeon, account of its

breeding-places and numbers,

31

490

INDEX

Pelagic animals, colours of, 193
Phasmidse, resemblance of, to sticks

and leaves, 203
Physiological selection, 180
Pickard-Cambridge, Rev. 0., on sexual

selection, 296 (note)
Pieridse, sexual diversity among, 271
Pigeons, varieties of, 89

domestic, derived from wild

rock-pigeons, 90
curious correlations in, 140
white eggs of, protective, 213
Pigs, great increase of, in South
America and New Zealand,
28
Pikermi, extinct animals of, 377
Pipits as illustrating divergence, 108
Planorbidse, variations of, 44
Plants, the enemies of, 16
variability of, 76
constitutional variation of, 94
colour relations of, 302
true mimicry rare in, 303
exotic rarely naturalised in

Europe, 356
dispersal of, 361
northern, in southern hemi-
sphere, 368
identical on summits of remote

mountains, 369
progressive development of, 397
geological development of (dia-
gram), 402
Plovers, recognition marks of

(figure), 221
Plumes, origin of accessory, 291

muscular relation of ornamental,
292
Poisonous fruits, 307
Porto Santo, rabbits of, 326
Poulton, Mr. E. B., on variable colours
of larvae and pupae, 196, 198
on concealments of insects by
resemblance to environment,
202
on protective form of Noto-

donta ziczac, 210
on inedibility of conspicuous
larvae, 237
Pouters, 90
Primulaceaa, variations of structure in,

79
Problem, the, before Darwin, 6

Problems in variation and heredity, 410
Progression in plants and animals,

395
Protection by terrifying enemies, 209
Protective colouring, variable, 195
of white-headed fruit -pigeon,

200
of African sun-birds, 200
of Kerivoula picta, 201
of sloths, 201

of larva of Sphinx ligustri, 202
of stick and leaf insects, 203
of caterpillars, 203, 205
of butterflies, 206
Ptilopus cinctus, protective colour of,

200
Pugnacity of birds with accessory
plumes, 294

R

Rabbit, use of white tail of, 218
Rapid increase of plants, 28
Raspail, M., on variability in a grass,

80
Rat, black, spread of, 34
Rattlesnake, use of rattle of, 262
Raven, why black in arctic regions,

191
Reciprocal crosses, 155
Recognition marks of herbivora, 218
of birds, 222

of tropical forest birds, 224
of insects, 226
Reproductive functions, susceptibility

of, 153
Reptiles, geological distribution of, 406
Rhinoceroses, evidence of evolution

afforded by fossil, 383
Rocks, all stratified formed in shallow

water, 344
Rocky Mountains, scarcity of mono-
cotyledons in, 401
Rodents, prevent Avoody vegetation in

the pampas, 23
Romanes, Professor G. J. , on useless

characters, 131, 139
on meaningless peculiarities of

structure, 140
on supposed absence of siimd-

taneous variations, 142
on physiological selection, 180

INDEX

491

Rook and crow, 2

Roses, Mr. Baker on varieties of, 77

Rubus, Bentham and Babington on

species and varieties of, 77
Rudiments and variations in man,

446
Runts, 91
Rutacese, variation of structure in,

79

s

St. Helena, destruction of forests by

goats, 17
St. Hilaire, M. Aug., variability of

Gomphia olesefolia, 79
Saxicola, divergence of character in
species of, 108
recognition marks of, 222
Scientific opinion before Darwin, 4
Scolopax, figures of tails of, 225
Scudder, Mr. S. H., on inedibility of
Danais archippus, 238
on fossil insects, 403
Seebobm, Mr. , on swamping effects of

intercrossing, 143
Seeds, bow dispersed, 306
how protected, 307
floating great distances, 361
dispersal of, by wind, 362
weight and dimensions of, 364
importance of wind-carriage of,

372
remarkable case of wind-carriage
" of, 373
Seiurus carolinensis, diagram of varia-
tion, 67
sp., habits of, 117
Selection, artificial, 84

by man, circumstances favour-
able to, 96
unconscious, 96
Selective association, isolation by,

171
Self- fertilisation of flowers, 321
Semper, Professor, on casting hairs of
reptiles and crustacea, 137
(note)
on direct influence of environ-
ment, 426
Sesiidse, mimicry by, 240
Sex colour and nests of birds, 277

Sex, colours characteristic of, 269
Sexual colours of insects, probable
causes of, 273
of birds, 275

characters due to natural selec-
tion, 283
diversity the cause of variation,
439
Sexual selection and colour, 274
by struggles of males, 282
neutralised by natural selection,

294-296
restricted to male struggles, 296
Shetland Islands, variety of ghost-
moth in, 270
Shrews and field-mice, internal varia-
tions of, 69
Shrikes, recognition marks of, 222
Similarity of forms of life not due to
similarity of conditions, 339
Singing of male birds, use of, 284
Skull of wolf, diagram of variations
of, 70
of Ursus labiatus, diagram of

variations of, 72
of Sus cristatus, diagram of
variations of, 73
Skunk an illustration of warning

colour, 233
Slack, Baron von, on protective

markings of sloths, 201
Sloth, protective colour and marking

of, 201
Snakes, mimicry of poisonous, 261
Snipe, tails of two species (figs.),

225
Sounds and odours peculiar to male,
how useful, 284
produced by peculiar feathers,
284
South America, fossil and recent

mammals of, 393
Species, definition of, 1, 2
origin of, 2, 6
Lyell on, 4
Agassiz on, 5
transmutation of, 6
Geoffroy St. Hilaire on, 6
Dean Herbert on, 6
Professor Grant on, 6
Von Buch on, 6
allied, found in distinct areas,
36

492

INDEX

Species, which vary little, 80

closely allied inhabit distinct

areas, 111
vigour and fertility of, how kept
up, 327
Spencer, Mr. Herbert, on factors of
organic evolution, 411
on effects of disuse, 413
on difficulty as to co-adaptation

of parts, 417
on direct action" of environment,
418
Sphingidse, protective attitudes of

larvse, 210
Sphinx ligustri, general resemblance

of larva to food plant, 202
Spider, alluring coloration of, 211
Spines, on origin of, 431

rarity of, in oceanic islands, 432
Spiny plants abundant in South Africa

and Chile, 433
Spots a primitive ornamentation of

animals, 289
Sprengel on flowers and insects, 309
Staphylinidse, protective habit of,

210
Sterility of mongrels, 165

correlated with colour, etc., 168
and natural selection, 173
of hybrids produced by natural
selection, 179
Struggle for existence, 14
among plants, 15
for life, illustrations of, 18
for existence on the pampas, 22
for life between closely allied

forms most severe, 33
for existence, ethics of, 36
how it acts among flowers, 328
Summary of facts of colouring for
protection and recognition,
227
Survival of the fittest, 11, 122, 123
Swainson, definition of species, 2
Swamping effects of intercrossing,

142
Sweden, destruction of grass by larva?

of moths in, 17
Swinhoe, Mr., on protective colouring

of a bat, 201
Symmetry, bilateral in colours of
animals needful for recogni-
tion, 217

Tails used as respirators, 136

Tapirs, distribution of, 352

Tegetmeier, Mr. , on feeding habits of
pigeons and fowls, 75
on sparrows and crocuses, 75
on curious correlations in
pigeons, 140

Tegumentary appendages and colour,
291

Thousand-fathom line divides oceanic
from continental islands, 347
the teachings of, 34S
map showing, 349

Thwaites, Mr., on spread of Lantana
mixta in Ceylon, 30

Tiger, use of stripes of, 199

Titmice as illustrating divergence,
107

Transformation of species of Crustacea,
427

Transmutationists, the early, 3

Travers, Mr. W. L., on effects of in-
troduced plants in New Zea-
land, 29

Trees, great variety of, in many
forests, 36

Trimen, Mr., on butterfly deceived by
its mimic, 245
on mimicry, 247

Tropical animals, why brilliantly
coloured, 299

Tropics, no proof of loAver tempera-
ture of, 369

Tropidorhynchi mimicked by orioles,
263

Trumpeter, 93

Tumblers, 91

Turbits and owls, 91

Tylor, Mr. A., on Coloration in Ani-
mals and Plants, 285

U

Ungulates, origin of feet of, 423
Use and disuse, effects of, overpowered

by natural selection, 435
Useless characters, 131
not specific, 132

INDEX

493

Useless specific characters, no proof

of existence of, 141
Utricularice inhabiting bromelias, 118

Vanessa callirhoe, small variety in
Porto Santo, 106

Variability of the lower animals, 42
of the Foraminifera, 43
of sea-anemones, 43
of land mollnsca, 43
of insects, 44
of lizards, 46
of birds, 49

of primary -wing-feathers, 51
of wings and tail, 53
of Dolichonyx oryzivorus, 55
of Agelseus phcenicens, 56
of Cardinalis virginianns, 58
of tarsus and toes, 60
of birds in Leyden Museum, 61
of Sciurus carolinensis, 67
of skulls of wolf, 70
of skulls of a bear, 72
of skulls of Sus cristatus, 73
of plants, 76
of oaks, 77

Variation, Lyell on, 4

in internal organs, 66
the facts of, 83
proofs of generality of, 85
of vegetables and fruits, 86
of apples and melons, 87
under domestication accords
with that under nature, 100
coincident not necessary, 127
and heredity, problems of, 410
Professor Geddes's theory of, 428
the cause of, 439

Variations of flowers, 88

of domestic animals, 88
of domestic pigeons, 89
conditions favourable to produc-
tion of, 98
beneficial, 143

Varieties, importance of, 41

of same species adapted to self
or to insect-fertilisation, 330

Vegetables, variation of, 86

Vegetation and reproduction, antag-
onism of, 428

Vertebrata, mimicry among, 261
geological succession of, 405

Vestiges of Creation, 3

Viola odorata, 2
canina, 2

Violets, as illustrating species, 2

Von Buch on species, 6

W

Wallace, Dr. Alexander, on absence
of choice by female moths,
275
Ward, Mr. Lester F., on progressive

development of plants, 398
Warning coloration, 232
Warning colours of marine animals,

265
Wasps and bees, mimicry of, 258

poisonous with warning colours,
287
Water-cress, chokes rivers in New
Zealand, 24
driven out by willows, 24
Water-ouzels, probable origin of, 116
Weale, Mr. Mansel, on protective

colours of butterflies, 206
Weeds of United States, 1 5
Weir, Mr. Jeuner, on deceptive re-
semblance of a caterpillar to
a twig, 204
on inedibility of conspicuous

caterpillars, 236
on birds disregarding inedible
larvse, 254
Weismann on progressive adaptation
of colours of larvas, 206
on non-heredity of acquired

characters, 440
and Galton's theories of heredity

almost identical, 443 (note)
on origin of the mathematical
faculty, 472 (note)
Weismann's theory of heredity, 437
Westwood, Professor, on variation of
insects, 44
deceived by a mimicking cricket,
259
White coloration of insular birds and

butterflies, 230
Whymper, Mr., his sensations when
falling on the Matterhorn, 38

INDEX

Willows, species and varieties of
British, 77

Wilson, Alexander, his account of
the j>assenger-pigeon in North
America, 31

Wind-carriage of seeds explains many
facts of plant distribution,
371

Wind-dispersal of seeds, objections to,
365

Wind-fertilised degraded from insect-
fertilised flowers, 324

Wings of stone-curlews (figure), 223
why small but useless are re-
tained, 416

Wit and humour, origin of factdties
of, 472

Wollaston, Mr. T. W., on variation
of beetles, 44
on small butterfly in Porto
Santo, 106

Wolves, varieties of in Catskill Moun-
tains, 105

Wood, Mr. J., on muscular variations,
447
Mr. T. W. , on variable colour-
ing of pupa? of cabbage butter-
flies, 197

Woodward, Dr. S. P., on variation of
mollusca, 43

Youatt, on breeds of sheep, 97
Young animals often spotted, 289

3

Zebra, markings for recognition and
protection, 220 (note)

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